A Brief History of the Philosophy of Mind


Part I

The Classical World

Plato observes that philosophy begins in wonder.  Science also begins in wonder, and all sciences, including psychology, were originally part of philosophy.  The early Greek philosophers were really philosopher-scientists who wondered about the essential nature of the universe.  Gradually, over the centuries, each science, beginning with astronomy, separated itself from philosophy to become an independent science.  Psychology remained within the fold of philosophy until the nineteenth century.  The first scientific psychologists, such as Wundt, Kulpe, and James, were also philosophers, often using their philosophical positions to support their psychological research and vice versa.

The first philosophical inquiries into the world were physical.  Philosophers from Thales to Democritus wanted to know what the universe was like, what were its basic constituents and its laws.  They laid the foundation for modern natural science: indeed, remarkable parallels exist between ancient Greek atomism and modern physics.  As psychology is a science, it owes a debt to these thinkers who started science.

The nature of philosophy changed, however, in the second half of the fifth century B.C. Philosophers stopped asking the questions of physics and began asking the questions of psychology.  The primary physical question is: what is the universe that people can know it?  The primary psychological question is: what is a person that he or she can know the universe?  No longer did philosophers seek to know the fundamental characteristic or matter, seeking instead to understand knowledge itself.  What is knowledge?  How do we acquire knowledge?  What is knowledge about?  This field of philosophy is called epistemology, from the Greek words episteme, knowledge, and logos, account or discourse.  Epistemology is naturally related to psychology, for it is people who know and people who learn.  Plato made epistemology the central concern of philosophy for two millennia.  Psychology, at least as it was founded, is trying to wed science to epistemology, to give scientific answers to philosophical questions.  The important psychological issues were originally philosophical, and so it is impossible to understand psychology historically without knowing about philosophy, especially epistemology.

We must not forget, however, that psychology wedded science to philosophy.  The first psychologists were philosophers; they were also physiologists.  Human beings as thinking, knowing creatures cannot be considered apart from humans as biological organisms.  Humanity knows, but humanity’s knowledge is the outcome of physiological sensation and central cortical processes.  Psychologists from the beginning have been aware of that, and so we cannot understand psychology without knowing about biology.  However, more space is given here to philosophy because with the exception of evolution – which has been of supreme importance in shaping twentieth century psychology – the important concepts, issues, and questions of psychology have come from philosophers, not from biologists.

The pre-Socratics

The Greeks had no word corresponding to “personality,” but they did have names for what we would call different components of personality.  First, there was psyche, the “breath of life” from which “psychology” derives, that leaves a person at death; we may interpret psyche as the vital principle of life that separates the organic from the inorganic.  Another part of personality was thymos, which seems to mean a motivational principle underlying both action and feeling.  Our own word (e)motion also expresses the idea that behavior must result from motivational arousal.  Finally, there was nous, the psychological organ for the clear perception of truth.

It is difficult for people to accept criticism of their ideas or to reflect critically on them.  Consequently, many systems of though are closed, that is, they do not criticize themselves against criticism.  We often find closed systems of thought in religion, for believers adhere to some great revealed Truth beyond human criticism; critics are called heretics and are often persecuted.  Political systems, too, may be closed.  The ancient Greek philosophers were the first thinkers to progress by employing criticism.  There, beginning with Thales of Miletus (flourished 585 B.C.E.), a tradition of systematic criticism whose aim was the improvement of ideas, came into being.  As the philosopher Karl Popper wrote (1965): “Thales was the first teacher who said to his pupils: “This is how I see things – how I believe that things are.  Try to improve upon my teaching.'”  Thales did not teach his ideas as a received Truth to be conserved, but as a set of hypotheses to be improved.   Thales and those who followed him sought change.  They knew that ideas are rarely right, that only by making errors and then correcting them can we progress.  Dogma enshrines error in concrete and makes progress impossible.  The critical attitude is fundamental to both philosophy and science, but it requires overcoming intellectual laziness and the natural feeling of hostility towards critics.  Founding a critical tradition was the major achievement of the Greek inventors of philosophy

The specific problem Thales addressed was the nature of reality.  (metaphysics)  Besides inaugurating a critical tradition, then, Thales also began a line of physical investigation.  In doing so he moved away from religious or spiritual interpretations of the universe toward naturalistic explanations of how things are constituted and how they work.  Thus Thales asserted that the world is within human understanding, for it is made up of ordinary matter and does not reflect the capricious whims of gods.  Thales proposed that although the world appears to be made up of many different substances (wood, stone, air, smoke, and so forth), there is in reality only one element – water – which takes on many forms.  Thales propose, the essential constituent of all things.  The name for the single element out of which all things are made was physis, and so those who followed Thales in searching for some such universal element were called physicists.  Modern physics continue the search, asserting that all the substances of common experience are really composed of a few elementary particles.

Anaximander of Miletus (flourished 560 B.C.E.) who accepted the concept of a physis but criticized Thales’ hypothesis that it was water.  He proposed the existence of an element that was not any recognizable element, being instead something less definite that could take on many forms.  He called his proposed physis the aperion, best translated as “the Indefinite.”  In turn Anaximander was challenged by his student Anaximenes of Miletus (flourished 546 B.C.E.) who proposed that the physis was air.  Anaximander also deserves notice for his shrewd observation on theories of evolution.

Xenophanes of Colophon (flourished 530 B.C.E.) broadened the critical and naturalistic traditions by his open assault on Greek religion.  Xenophanes maintained that the Olympian gods were simply anthropomorphic constructions, behaving just like human beings, even lying, stealing, murdering, and philandering.  Xenophanes argued that if animals had gods they too would make them in their own images, inventing lion gods, cat gods, dog gods, and so on.  Xenophanes’ critique is the beginning of the ancient struggle between scientific naturalism and religion that reached its greatest crisis when Darwin proposed the theory of evolution in the nineteenth century.

More directly influential on later philosophers, especially Plato, was Pythagoras of Samos (flourished 530 B.C.E.).  Pythagoras was an enigmatic figure, both a great mathematician and a religious leader.  He is most famous for the Pythagorean theorem, and he also formulated the first mathematical law of physics, expressing the harmonic rations of vibrating strings of different lengths.  Mathematics, however, was more than just a tool of science for Pythagoras.  It was also a magical key to the cosmos.  Pythagoras founded a secret religious sect devoted to numbers which believed: “Everything that can be known has a number; for it is impossible to grasp anything with the mind…..without this [number]” (Freeman, 1971).  In psychology, Pythagoras drew a sharp distinction between soul and body.  Not only could the soul exist without the body, but, going further, the Pythagorean’s considered the body a corrupting prison in which the soul was trapped.  Plato was greatly influenced by the Pythagorean’s.  He too viewed the soul as a pure knowing entity thrust into a corrupting body.  His theory of knowledge held that sense perception, depending as it does on the corrupt body, is inherently untrustworthy.  Instead, the soul’s reason should seek abstract knowledge of mathematical purity.  Finally we come to Alcmaeon of Croton (flourished 500 B.C.E.), because he foreshadows the founding of psychology.  Alcmaeon was a physician who practiced the first dissections.  He dissected the eye and traced the optic nerve to the brain.  Unlike later thinkers, such as Empedocles and Aristotle, Alcmaeon correctly believed that sensation and thought occur in the brain.  Alcmaeon’s work hints at the founding of psychology, the attempt to answer philosophical questions about reason by using scientific methods borrowed from physiology.  In most founding psychologists, including Wilhelm Wundt, Sigmund Freud, and William James, we will find the figure of Alcmaeon, the physician turned empirical philosopher.

Being and Becoming:

Heraclitus of Ephesus (flourished 500 B.C.E.) asserted that the universe was in constant state of flux.  His most famous aphorism was that no one ever steps in the same river twice.  Nevertheless, Heraclitus also believed that although change is the only constant, it is lawful and not capricious.  Regulating change is a dynamic universal harmony that keeps things in an equilibrium of balanced forces.  Thus what truth philosophy and science may meet will be truth about change and not about static things.  The philosophy of being was first stated by Parmenides (flourished 475 B.C.E.).  Parmenides sharply distinguished a Way of Seeming (appearances) from a Way of Truth (reality).  Since, for Parmenides, Truth was eternal and unchanging, the philosopher concluded that change is an illusion based on our faulty senses.  In reality there is no change.  This changeless reality had to be grasped by reason and logic, and Parmenides was the first philosopher to present his arguments as logical deductions from intuitively plausible premises.  Parmenides is thus the founder of rationalism.

Empedocles of Acragas (flourished 450 B.C.E.), who may be regarded as the founder of empiricism.  Building on the ideas of Alcmaeon, Empedocles tried to develop a theory of perception that would justify our common sense reliance on our senses.  Empedocles stated that objects emit ‘effluences’ that are sense-modality specific copies of themselves.  Today we know that smell works this way; our noses respond to certain molecules given off by some things.  Empedocles thought this true of all kinds of perception.

Empedocles’ views are characteristically empiricist, claiming that we know reality by observing it, specifically by internalizing copies of objects.  Thought can create nothing new, being able only to rearrange the atoms of experience.  And Empedocles’ conclusions show why empiricists have generally contributed more to psychology than rationalists have.  The empiricist must show how the senses work in order to justify our using them in seeking the truth.  This necessarily requires developing psychological theories of sense-functioning.  The rationalist, on the other hand, simply denies the validity of sensory information, and so can ignore problems of empirical psychology as philosophically irrelevant.

The last classical philosophers to be concerned primarily with the nature of physical reality were Leucippus of Miletus (flourished 430 B.C.E.) and his better known student, Democritus of Abdera (flourished 420 B.C.E.).  After them, philosophers turned to questions about human knowledge, morality, and happiness.  As the name of their school implies, the atomists’ proposed an idea that has proven immensely fruitful in physics: that all objects are composed of infinitesimally small atoms.

Atomism can be metaphorically extended to psychology, where it has proved to be the most durable of psychological assumptions.  Psychological atomism says that complex ideas such as “cathedral” or “psychology” can be analyzed as collections of simpler ideas, or even of sensations, that have been associated together.  This assumption has been an integral part of empiricist theories of the mind and it still, in some form, underlies all psychological systems except Gestalt psychology.  The atomists pushed their hypothesis to its limit.  They supported materialism, determinism, and reductionism.  A favorite motto of Democritus was that only “atoms and [the] Void exist in reality.”  There is no God and no soul, only material atoms in empty space.  If only atoms exist, then free will must be an illusion.  Leucippus said, “Nothing happen at random; everything happen out of reason and by necessity.”  The soul and free will are illusions that can be reduced to the mechanical functioning of our physical bodies.  Democritus wrote, “We know nothing accurately in reality, but only as it changes according to the bodily condition and the constitution of those things that impinge upon [the body]” (Freeman, 1971).

Like Empedocles, Democritus proposed a materialistic account of perception and thinking.  Indeed, Democritus’ theory is only a modification of that of Empedocles.  Democritus did see the fatal flaw in his theory of sense perception in that we have no way of knowing if our sense perceptions are accurate of truly knowing objects and knowledge.  We sill latter see how this problem became a sticking-point for the eighteenth century empiricists, Locke, Berkeley, and Hume.    It also has a moral pitfall in that it reduces an ethical doctrine down to materialistic reductionism which equates leading a life of hedonism and living a life in the pursuit of pleasure and reduction of pain.  This later troubled the eighteenth century ethical philosophers deeply as well.

Philosophy’s shift of focus from the nature of physical reality to the nature of man was expressed most forcefully by the Sophists.  Their famous motto was enunciated by the greatest Sophism Protagoras (approximately 490-420 B.C.E.): “Of all things the measure is man, of things that are that they are, and of things that are not that they are not” (Sprague, 1972).  The center of concern became man and his needs and not the physical world or the gods.  Protagoras’ motto reflects a humanistic relativism: man is the measure of all things.  this aphorism has a range of meanings.  The narrowest interpretation says that one is the best judge of one’s own experience.  Tow people may enter the same room, yet one may experience the room as warm, the other cool, if the former has been out in a blizzard and the latter downstairs stoking the furnace.  Neither perception is incorrect; each is true for its perceiver.  Generalizing this perceptual relativism brings us to a broader meaning of Protagoras’ idea: cultural relativism.  The Sophists tended to be materialists like Democritus, considering pleasure and pain to be the only guide to conduct.  Pleasure and pain are individual sensory experiences, so it follow that ethically each person is the only judge of what is right for her or him.  Any attempt to lay down general rules of conduct is necessarily arbitrary, for the law-giver knows only his or her own pleasures and pains.  Nevertheless, the Sophists recognized that law was necessary for the survival of human communities and accepted a cultural relativism by which any person living in one culture had to live by the rules of that culture but should not attempt to impose that culture’s rules on people from other cultures.

Finally, at its greatest level of generality, “man is the measure of all things” is a statement about the universe.  There is no permanent, enduring Truth, no divinely sanctioned law, no eternal trans-human code of values.  The measure of things in not God or abstract, scientific truth, but human beings, their needs, and their search for happiness  This view is central to humanism and offers a philosophy of becoming quite different from that of Heraclitus.  Like Democritus’ hedonism, the Sophists’ humanistic relativism is offensive to those who see in it a recipe for moral anarchy and a denial of enduring Truth.  In dialogue after dialogue, Plato’s Socrates defeats the Sophists, who appear as characters in many of Plato’s dialogues.  Out of Plato’s attempt to refute relativism came a powerful philosophy of being, classical rationalism.

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Plato (428 – 348 B.C.E.)

Plato divided the soul into a tripartite model.  First there is the immortal, rational soul, located in the head.  The other two parts are mortal.  The spirited or courageous soul, oriented to winning honor and glory, is located in the chest, and the passionate or appetite soul, concerned with bodily pleasure, in the belly.  The rational soul is akin to the Forms and to knowledge’ the perishable (mortal) souls are tied to the body and hence are only capable of opinion.  It is the duty of the rational soul to control the desires of the other two, as a charioteer controls two horses.  The passionate soul was viewed by Plato as particularly troublesome and requiring great restraint by reason.  Centuries later, we find a similar idea in Freud who also stressed the “primacy of reason” over instinctual drives.  Plato was clearly a mind-body dualist who said that a person is defined by his rational mind, the body being a disturbing tomb in which the soul finds itself incarnated and which it operates like a puppet.


As might be expected, Plato, especially in his early and middle works, takes a dim view of pleasure.  Seeking pleasure and avoiding pain, man’s obvious drives, are things of the body that serve only to debase the rational mind and hinder its contemplation of the Good.  All forms of sensation, including pleasure, were seen as unavoidable evils.  In his later writings, however, Plato modified this extreme view.  Some pleasures, such as the aesthetic joy found in beauty, he now considers healthy and he rejects the purely intellectual life as too limited.  His view of motivation becomes Freudian:  We have within us a stream of passionate desire which can be channeled to any of the three parts of the soul, into the pursuit of physical pleasure, honor, or philosophical knowledge and virtue.  Our drives can motivate either the pursuit of transitory pleasure or the philosophical ascent to the world of the Forms.

Physiology and Perception:

Plato’s physiology is quaint to our ears.  He said, for instance, that the function of the liver was to display images sent by the rational soul to the passionate soul; these images were later erased by the pancreas.  Since Plato distrusted perception, he said little about the empirical science  of physiology.  He often just records traditional Greek views.  Of vision, for example, he said that we see because our eyes throw out visual rays which strike objects in our line of sight.  This idea persists in modern language in such phrases as “he threw her a glance,” and this theory dominated optical thinking for centuries after Plato.


Plato was the first great nativist, for he believed that all human knowledge is innate, that is, present at birth.  In his more extreme moments, Plato believed that this knowledge can be revived only through dialectic and contemplation, giving no role to sense perception.  Elsewhere, however, Plato proposes an account of learning – his theory of recollection – that resemble certain modern theories, for example, Noam Chomsky’s nativist account of language acquisition.  Perceived objects, of course, resemble the Forms they partake of, and the resemblance, especially if aided by teaching, can stimulate our rational soul to remember what the Forms are like.  Put in modern terms, perceptual input arouses and develops innate cognitive mechanisms.  At the same time, Plato provides the basis for the doctrine of associationism, later a fundamental part of empiricist philosophy.  Sensible objects remind us of the Forms either because they are similar to the Forms, or because the two objects of ideas have been frequently associated in our experience.  These are the two of the fundamental laws of association – resemblance and contiguity – central to many later psychological systems.

Development and Education:

Plato believed in reincarnation.  At death the rational soul is separated from the body and attains a vision of the Forms.  Then, depending on the degree of virtue present in one’s previous life, one is reincarnated somewhere on the phylogenetic scale.  When the soul is thrust into a new body full of animal sensations and desires, it becomes completely confused and must adapt.  This confusion explains why knowledge of the Forms is not present in infants.  It is the purpose of education to help the rational soul gain control of the body and of the other parts of the soul.  Education has three phases.  First, infants must be soothed and rocked to master their inner chaos.  Then elementary education in gymnastics, rhetoric, and geometry gives the child mastery of the external world.  Finally, for those who are capable, higher education is philosophy leads one to knowledge of the Forms.  This education is especially rigorous and exacting and was meant to produce the rulers of society.  Plato’s psychology is fragmentary and incomplete.  The first systematic psychology was worked out by his student Aristotle, who had a higher regard for perception and empirical science than had his teacher.

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Aristotle (384 – 322 B.C.E.)

Aristotle was the first professor.  Plato wrote dramatic dialogues in which Socrates’ flashes of insight illuminated philosophical and moral problems.  Aristotle wrote prosaic treatises.  He was the first to systematically “review the literature” of earlier thinkers.  Instead of being led by intuitive insight, he was guided by order, method, and the syllogistic logic he invented.  Plato’s rationalism forced him to adopt fantastic ideas, such as the Forms, which do violence to common sense.  But Aristotle’s careful, empirical attitude never strayed far from common sense, and his errors were usually simple and factual, such as his belief that the heart was the seat of the soul, which includes the mind.  Plato created a magical world of disembodied Forms and mysterious forces of participation.  Aristotle’s world was one founded on common sense, in which heavy objects fall faster than light ones.


The soul is the form(or formal cause), essence, and actuality of the person.  The soul is what defines an animal – a cat is a cat because it has a cat’s soul and behaves like a cat.  A human being is human by virtue of possessing a human soul and hence acts human.  The soul is thus the essence of the animal.  Finally, it is the actuality of a body which potentially has life.  Without soul a body is dead; with soul there is life.  The potential for life in a creature, therefore, is actualized by the soul.  In addition, the soul is the efficient cause of bodily movement, for it causes movement to happen.  It is also the final cause, for the body serves the soul.  To summarize, of any animal the material cause is the body of which the animal is made, while the soul is efficient cause of motion, formal cause that defines the animal’s essence, and final cause, the purpose of the body.  ~~ see Aristotelian Four Causes

What is the relation of soul and body?  Aristotle, a biologist, took a naturalistic view of the mind-body problem.  The soul, with the exception of one part, is inseparable from the body.  His view resembles what is today called the dual aspect position: there is only one material reality, body, but it has two aspects, physiological and mental.  see Dual-Aspect approach to the mind-body problem  Soul is the form of body and can no more be separated from its material embodiment than the form of the Venus de Milo can be separated from the marble it is made of, although we can discuss them separately, considering either the marble or the shape alone.  Aristotle put it this way in De Anima: “That is why we can wholly dismiss as unnecessary the question whether the soul and the body are one: it is as meaningless to ask whether the wax and the shape given to it by the stamp are one…..”  see Aristotle De Anima / On the Soul

Aristotle was not a dualist.  He rejected Plato’s dualism and would have rejected Cartesian Dualism.  He is not a materialist reductionist, however.  Soul cannot be reduced to body, even though there is only one matter, for we can separately discuss physiological and psychological functioning.

Structure of the Rational, Human Soul:

According to Aristotle, gaining knowledge is a psychological process that starts with the perception of particulars and ends with general knowledge of universals.  Aristotle is in a sense the first information processing psychologist: we receive information from the senses, process and store this information, and act on it to yield knowledge, solve problems, and make decisions.  Aristotle’s analysis of the soul can be represented by an information processing flow-chart



Part II

The Middle Ages (476 – 1453 A.D. )

The middle ages was the crucible in which our modern world was formed; the Renaissance was the first self-consciously modern period.  The medieval period saw the beginning of constitutional democracy, romantic love, individualism, and experimental science.  During the Renaissance learning and scholarship left the confines of the Church to become again the property of lay society concerned with humanity’s nature and needs rather than God’s.  Although tradition sets the date of the end of Classical civilization at A.D. 476, something like the medieval way of life began during the Roman Empire in the late third and fourth centuries.  Because of an economic decline, small farmers became legally tied to the land, a state which evolved into serfdom.  As the control of Rome over her provinces loosened, local autonomous leadership grew which led to feudalism.  The breakdown of the Roman world was evident as barter economy began to replace the money economy of the Empire, communication broke down, the Imperial army became more and more a mercenary army of barbarians rather than a voluntary army of Roman citizens, populations declined, and the Eastern Empire with its own Emperor and capital at Constantinople leached treasure and resources from the European, or Western Empire to preserve its own superior way of life.

The crises were compounded by an extraordinary movement of barbarians into the empire.  Early settlers had often come peacefully into the Empire, but later invasions were bloody and destructive.  Rome itself was sacked in Augustine’s time; the Emperor Romulus Augustulus who fell in 476 was himself only a barbarian usurper.  The movement of northern peoples from Goths to Vikings, which finally tore the Empire asunder, continued to almost A.D. 1000.

This extended period of transition from classic to medieval times, from before 475 to about 1000, is sometimes still called the Dark Ages, but is better called the Early Middle Ages.  Although creative thinking declined, there were periods of intellectual development, most notably the Carolingian Renaissance under Charlemagne (768 – 814, not to be confused with the Italian Renaissance of the fifteenth and sixteenth centuries).

St. Augustine (354 – 430)

Augustine ((354-430) was the last great classical philosopher; he was also the first great Christian philosopher.  Augustine wanted only to know God and the soul, and used faith to justify belief.  Medieval humanity turned away from the observable world, full of pain and turmoil, and concentrated on heaven and the soul both of which could be known through introspection.

The Middle Ages sought a grand synthesis of all knowledge.  Since all knowledge was of God, the soul, and the spiritual world, it was believed that knowledge, tradition, and faith could be synthesized into a single grand authoritative picture of the universe.  The summit of such efforts was the Summa Theologica of St. Thomas Aquinas (1225 – 1274).  This belief, too, broke down after 1300.

For a detailed examination of how the bible crosses the holly lands into Islam going through the translations from Hebrew into Greek into Arabic before being translated into Latin, see the Gnostic Quill article on the history of the bible.  The History of the Bible

The High Middle Ages saw an intellectual renaissance as the works of Aristotle and his Muslim commentator, such as Avicenna (Ibn-Sina) (c. 980—1037), and other Greek works poured into the West through Spain, Sicily, and Constantinople.  Aristotle’s philosophy was naturalistic and as such was restricted by the reigning, mystical Augustinian  establishment of the time.  Aristotle brought a fresh, nonreligious approach to knowledge and humanity, an approach that was reconciled with Christian faith only with difficulty.  Thomas Aquinas, who synthesized faith in God’s work and reason as found in Aristotle’s philosophy, only narrowly escaped the charge of heresy.  The union of Christ and Aristotle, impressive though it was, was relatively sterile.  The future belonged to those who, like William of Ockham, divorced faith from reason and pursued only the latter.

St. Thomas Aquinas (1225 – 1275)

St. Thomas Aquinas adopted Aristotle’s system and showed that it was not incompatible with Christianity.  In doing so he stood Aristotle on his head.  Where Aristotle stays close to nature and is silent on God, Aquinas reorients everything to depend on and reveal Him.  To reconcile philosophy and theology, Aquinas distinguished sharply between them, limiting a person’s reason to knowledge of the world of nature.  Aquinas thus accepts Aristotle’s empiricism and the consequence that reason can know only the world, not Bod.  God can be known only indirectly, from His work in the world.  This is an important moment in the evolution of Western thought.  Aquinas is saying that philosophy and religion are separate, that while they are not incompatible, they do not connect.  This division finally destroyed the medieval synthesis Aquinas worked so hard to achieve.  However, Aquinas’ philosophy and theology are, in practice, if not in theory, intertwined; reason and revelation do make contact.  But later thinkers pursued his division of reason and faith to its logical conclusion and destroyed theological metaphysics while giving birth to science.

Science has displaced religion as the centerpiece of intellectuals of the modern world.  Scientific knowledge is taken as the model for all knowledge.  In the climate of persecution by the Church, their began a separation in the domains of faith and reason.  Only by asserting that the two were separate, that one did not bear on the other, could naturalism be defended as innocuous to Christianity.  It was a move that failed in Islam, where philosophy and science were stamped out after promising starts.  Europe, however, with its many nations and kings, was too heterogeneous to succumb to dogmatic repression.

William of Ockham (1290 – 1349)

William of Ockham whose contribution was to open up for psychological analysis what had previously been reserved to metaphysics.  Medieval philosopher confused psychology and ontology, the study of the nature of being or existence.  As did Plato, most medieval thinkers believed that there must be something real corresponding to each mental concept.  For Plato they were the Forms; for Aristotle they were real essences; for medievalist’s they were Ideas in the mind of God.  For the Greeks and medievals, the only real knowledge was knowledge of universals; indeed, it was asserted that the rational soul, or intellect, had knowledge only of universals, not of particular things.  Following Aristotle, they held that the only certain knowledge was what could be deduced from universal propositions.  This attitude existed even in Aquinas.  Although he described the process of abstraction as the way to universal knowledge, and although he held that the intellect knows only what is derived from the senses, he still maintained that the abstracted essences were metaphysically true, that they corresponded to hold Ideas.

Ockham changed all this by substituting psychology for metaphysics.  He asserted that all knowledge begins with “intuitive cognition,” which is direct, infallible acquaintance with some object in the world.  The intellect is not restricted to knowing only abstract images; what it knows first is objects and their qualities.  Intuitive cognition does not yield mere opinion, as Plato held, but yields knowledge of what is true and false.  From such knowledge it may go on to “abstractive cognition” of universals.  But these universals exist only as mental concepts, having no existence outside the mind.  These abstract concepts may be either true or false; for example, one may form the concept of a unicorn, which does not exist.  Abstractive cognition is thus wholly hypothetical.  The touchstone of reality and truth is intuitive cognition.  Ockham discarded the metaphysical problem that bedeviled Plato, Aristotle, and the medievals, namely how can each individual participate in a transcendent essence or form, and substituted the psychological question, how do we form universal concepts given that we have certain knowledge only of individuals?  His answer was that the mind notes similarities among objects, and, based on the similarities, it classifies objects.  Thus, universals are logical terms that apply to some objects and not others an which indicate relations among objects.  For Ockham, universals are a psychological problem rather than an ontological one.

Ockham’s analysis of knowledge successfully separated Reason and Revelation.  But precisely because revelation was not something known by Reason, minds turned to study the natural world, and religion became less and less important to European intellectuals, until, by the Age of Enlightenment, Revelation was openly rejected and deism or atheism adopted.  The immediate result of Ockham’s Ideas in the fourteenth century was an increased interest in science.

We can trace the modern scientific attitude back to Robert Grosseteste (1168 – 1253) and Roger Bacon (1214 – 1292), both English Franciscans like William of Ockham.  Grosseteste and Bacon both conducted experiments in optics because of their Platonic-Augustinian belief in the primacy of light among the world’s elements.  Both also stressed the role of mathematics in reaching an understanding of nature.  This belief is most important, for mathematization has been the touchstone of science from Galileo through Newton to Einstein.  Immanual Kant was to deny psychology scientific status partly because he believed the mind could not be studied mathematically.


Part III


The two centuries after 1600 were literally revolutionary.  The period begins with the Scientific Revolution of the seventeenth century and closes with political revolutions in colonial America and monarchical France.  The scientific and philosophical revolutions laid the basis for the political.  In broad historical terms these centuries witnessed the crystallization of the Western world we know today.  From 1600 – 1700 sees the establishment of modern science and the reconstruction of philosophy on new (yet familiar) lines.  From 1700 – 1800 the era of Enlightenment see the principles of science and reason applied to human affairs, including the study of human mind and behavior.

The scientific revolution may have said to begin in 1453 with the publication of Nicholas Copernicus’ Revolution of the Heavenly Orbs which proposed that the sun, not the earth, was the center of the solar system.

René Descartes (1596 – 1650)

René Descartes was a Renaissance man.  In three areas his influence has been deep and lasting: in his reformulation of rationalism, in his mechanical concept of the world, and in his dualist concept of humans.  Descartes was a skeptic but did not accept either the skeptics’ belief in the unattainablilty of knowledge or their low estimation of human reason.  Descartes found he could doubt the existence of God, the validity of his sensations, the existence of his body , (truth from doubt).  He continued in this way until he found one thing he could not doubt, his own existence as a self-conscious, thinking being.  One cannot doubt that one  doubts, for in doing so, one realizes the very action supposedly in doubt.  Doubting is an act of thinking, and Descartes expressed his first indubitable truth as the famous “Cogito, ergo sum”: I think, therefore I am.

Descartes was not the first to prove his own existence from mental activity.  St. Augustine had said, “If I am deceived, I exist,” and Parmenides had said, “For it is the same thing to think and to be.”  So we may place Descartes in the introspective rationalist tradition: truth is primarily evident in me, in my self-consciousness, my thinking.  After Descartes,d however, introspection became the major philosophical tool of rationalist and empiricist alike.  Philosophers differed about what they found in the mind, but they all looked to it for truth.  After Descartes, therefore, philosophy became increasingly psychological, seeking to know the mind through introspection, until tin the nineteenth century psychology is founded as the scientific, rather than armchair-philosophical, study of consciousness known through introspection.

Descartes proposed a dualism of mind and body seen as quite distinct entities, one physical (the body), the other nonphysical (the mind).  These two entities interact: the mind acquires information about the material world through the senses; the desires of the body are felt in consciousness, while the mind may direct the actions of the body.  Descartes sought to account for as much of the mind as possible on materialist, mechanical terms within the sphere of science, reserving only self-consciousness at most to philosophy.  Descartes gave great impetus to the assimilation of mind to mechanical science.  In the eighteenth century we will find complete mechanistic psychologies.  Descartes if finally a paradoxical figure.  In his emphases on reason as oposed to perception, on innate ideas as opposed to experience, on absolute truth as opposed to relativism, he is a rationalist.  However, in his mechanical view of the world and the human body his psychology would ultimately support empiricism and behaviorism.

Baruch Spinoza (1632 – 1677)

Spinoza’s philosophy begins with metaphysics and ends with a radical reconstruction of human nature.  Spinoza argued that God is essentially nature.  Furthermore, nature is entirely deterministic.  Spinoza extended his deterministic analysis to human nature.  Mind is not something separate from body, but is produced by brain processes.  Mind and body are one, but may be viewed from two aspects, as physiological brain processes or as mental events-thoughts.  Spinoza did not deny that mind exist, but did see it as one aspect of a fundamentally material nature.  Thus for Spinoza mental activity is as deterministic as bodily activity.  Spinoza rejected Cartesian dualism, and so for him there is no problem of interaction.  We feel we are free, but this in only an illusion.  Spinoza’s account of responsibility thus calls for a psychological science to unravel the causes of human behavior, and bears a striking resemblance to B.F. Skinner’s.  Spinoza’s account of memory, which says that ideas experienced together become mechanically linked also resembles later conditioning theories which associate stimulus and response.

Gottfried Wilhelm Leibniz (1646 – 1716)

Leibniz was a mathematician, logician, and metaphysician.  His answer to the dualistic problem of Descartes became increasingly popular over the next two centuries.  Descartes had said that mind and body interact.  However, it was unclear how spirit could act on matter and vice versa, leading to a view called occasionalism in which God saw to it that when a bodily event occurred, so did a mental event and vice versa.  Leibniz proposed an answer which has since been called mind-body (or psychophysical) parallelism.  Consciousness (mind) mirrors exactly what happens in the body, but only because of God’s pre-established harmony, not because of a causal connection.

On the other side of the English Channel modern empiricism was being founded.  In England there was a very different atmosphere, less heavy with metaphysics and more concerned with things as they are.  The empiricists are more descriptive in their approach to the mind.  Descartes, Spinoza, and Leibniz all wanted to improve the mind by propounding some method to escape error.  The empiricists were more interested in how the mind ordinarily works rather than how it ideally should work.

Thomas Hobbes (1588 – 1679)

Hobbes believed that all knowledge is ultimately rooted in sense perception.  In his work, Leviathan (1651), Hobbes wrote: “Understanding is nothing else but conception caused by speech.”  Further, he states that “Children are not endowed with reason at all, till they have attained the use of speech.”  Hobbes was the first in the long, and still living, line of British philosophers who equate right thinking with right use of language.  For psychology, this is an old and unresolved issue: whether thinking is overt or covert speech, or whether speech merely dresses up abstract concepts.  Hobbes clearly argued the former.

John Locke (1632 – 1704)

Unlike the rationalist Descartes, who sought ultimate Platonic Truth, Locke wanted to understand how the human mind actually works – the sources of its ideas, and the limitations of human knowledge.  Thus Locke’s epistemology is really a psychology, for his emphasis is on how the mind knows rather than on what it knows.  Locke thus brought the scientific spirit to philosophy, shearing off metaphysics, to say what can be empirically known about the human mind.  Locke states the empiricist principle that knowledge derives from experience alone.  He uses the famous simile for the mind of the ‘Tabula rasa’ (a Latin phrase often translated as “blank slate” ), or piece of white paper, on which experience writes ideas.

For Locke the mind was not merely an empty room to be furnished by experience, but was rather a complex information processing deice prepared to convert the materials of experience into organized human knowledge.  Direct experience provides us with simple ideas, which are then elaborated and combined by the mental machinery into complex ideas.

One can conclude that the differences between Locke the empiricist and Descartes the rationalist were primarily differences of emphasis.  Both wanted to transcend sterile scholastic philosophy; both tried to do so by examining the human mind.  Descartes was more the captive of the past, still searching with pure reason for transcendent truth.  Locke points more to the empirical future.  He recognized the limits of human knowledge and reason; indeed one reason for writing the Essay was to show what humanity could hope to know so that only fruitful questions  might be pursued.  In some ways, Locke was less empiricist than his predecessor Hobbes.  Hobbes said we think in our acquired language, that words are ideas.  Locke insisted that words are only signs of ideas.  Thus for Locke reason comes first, and then is framed in conventional words.  For Hobbes, more radically, one cannot think without acquiring language; reason come second.

Bishop George Berkeley (1685 -1753)

Like Locke and Descartes, Berkeley as a philosopher wanted to refute skepticism, while as a deeply religious man he wanted to refute the Newtonian materialism that imperiled faith in God.  He saw the problem of Locke’s view that our knowledge is about ideas ultimately rooted in sensation from a skeptic’s point of view.  Locke believed in the existence of “real” objects that exist beyond our perception and which have unobservable properties.  But the skeptic may ask, if ideas are the objects of our knowledge, if in fact what we perceive are ideas of objects, how can we be sure our ideas truly correspond to the “real” material objects?  In Berkeley’s view, Locke was helpless to refute the skeptic.

Berkeley was the problem to be Locke’s belief in matter as something apart from perception.  For example, as I am sitting here writing, I know my pen exists because I sense it.  But when I lay it down and leave the room, what grounds have I for asserting that it still exists?  All I can say is that if I went back I would see it, or if someone else looks for it, he or she will see it.  Ultimately then, I know the pen exists only because I see it.  And in fact, said Berkeley, the pen exists when it is perceived.  Berkeley’s famous motto is “Esse est Percipi”: to exist is to be perceived.  Berkeley thus refuted skepticism by an astoundingly simple assertion.  As Locke said, all we know are our ideas, but Berkeley adds that there is no permanent material reality apart from our perceptions.

David Hume (1711 – 1776)

Beginning with his A Treatise of Human Nature (1739), Hume strove to create a total naturalistic “science of man” that examined the psychological basis of human nature.  In opposition to the rationalists who preceded him, most notably René Descartes, he concluded that desire rather than reason governed human behavior.  He also argued against the existence of innate ideas, concluding that humans have knowledge only of things they directly experience.  He argued that inductive reasoning and therefore causality cannot be justified rationally.  Our assumptions in favor of these result from custom and constant conjunction rather than logic.  He concluded that humans have no actual conception of the self, only of a bundle of sensations associated with the self.

Thomas Reid (1710 – 1796)

Reid reasserted the use of common sense.  Reid found Hume’s conclusion offensive to common sense, as simply too absurd to be believed.  We all have secure knowledge of our world and never become skeptical unless prodded by abstruse philosophy.  He explicated common-sense by an analysis of the ordinary language which embodies it.  This approach is similar to that of modern ordinary-language philosophy.  Reid also anticipated two related ideas that grew out of act psychology and are later found in the Gestalt movement.  Reid said experience is not a compound of simple sensations.  Our primary experience is on complex impressions, to use Hume’s term.  Reid acknowledged that one can analyze complex impressions into simple ones, but denied that the complex ideas are formed by learned associations.  As the Gestalt psychologists would say, a triangle is composed of three lines, but we always experience it as an organized whole – the triangle.  Reid derived from this observation the conclusion that perception is always meaningful.  Concepts are mental symbols that stand for something real.  Perception is thus like language.  Complex experience cannot be reduced to atomic sensations without robbing it of something vital – its meaning.

Reid, like Hume, wanted a science of human nature conducted along Newtonian lines.  However, his extensive nativism coupled to his claim that the God-given first principles cannot be revised, puts his system outside psychology as a science.  The hypotheses of any science must be open to proof, test, and revision.  A similar criticism may be directed at Hume.  By replacing metaphysics with psychology, he made psychology into a metaphysics, a set of necessary principles underlying the other sciences.  Again, there could be no revision; Hume opened no paths for scientific investigation.  Both Reid and Hume elevated psychology to a central place among human concerns; both enunciated general principles and embodied certain attitudes that shaped psychology later; but neither was really a psychologist.  Both used psychology to pursue philosophy, their first concern.

Immanual Kant (1724 – 1804)

Kant was Hume’s empiricism as undermining certain knowledge and as threatening the absolute achievement of Newton’s physics.  As a result, Kant tried to rescue metaphysics.  He realized that the old speculative metaphysics about God and man’s spiritual substance was dead, and in fact Kant proved that it had always been an illusion.  However, Kant could not accept Hume’s merely psychological analysis of knowledge, for it only said we have a tendency to form general conclusions based on association.  Kant wanted to prove the validity of human knowledge quite apart from any empirical facts about human habit formation.  He thus reasserted the claim of philosophical metaphysics over psychology to be the foundation of the other sciences.

Kant’s answer to Hume bears a strong resemblance to Reid’s.  What we have knowledge of, in Kant’s term, is phenomena.  The objects of science, such as planets or balls rolling down inclined planes, are found in human experience.  Kant argued that experience is organized by the inherent nature of human perception and thinking.  For example, in our experience every event has a cause.  Why?  According to Hume, belief in causation is something learned, primarily by association.  But for Kant, Hume’s account undermined the absolute truth of causation; a mere habit cannot be absolutely true, as required by the Newtonian physics Kant took as his model of human knowledge.  Belief in causation therefore cannot derive from habit, but from something inherent in human thinking.  The world as we experience it, phenomena, must be such that every event has a cause, for this is the only way we can conceive the world.  Our experience will never violate causality because we are so constructed that every experienced event has a cause.  The Newtonian assumption of universal causality can never be falsified, and it is therefore absolutely and necessarily true – at least as regards phenomena.

Outside phenomena are what Kant called noumena, or things-as-they-are.  In the noumenal world there may be uncaused events, and in fact Kant assigned human moral freedom to the noumenal realm.  However, as noumena affect us to produce phenomena, all events are perceived to be caused.  Thus, according to science all behavior is caused, for science rests on phenomena; but man may very well be noumenally free – in fact must be free if moral responsibility is to have any meaning.  Kant built in many inherent principles of understanding which structure our phenomena – from time and space as preconditions of sensation to concepts of causality and existence.

What Kant tried to do, in his own words, is to carry out a “Copernican Revolution” in epistemology.  Previous empiricist philosophers assumed that humans have knowledge because objects impose themselves on understanding, which conforms itself to them.  Hume’s philosophy is the end point of this assumption.  For Hume, events in the real world are regular because of the laws of nature, and these regularities register themselves in our mind as habits.  But Hume’s view leads to at least moderate, if not total, skepticism, which Kant (like to many others) found appalling.  So, Kant decided to “revolutionize” philosophy by a new assumption – that objects conform themselves to our understanding.  We are endowed with certain qualities of perception and thinking which impose themselves on experience to create the objects of knowledge about which science makes true statements.  For the empiricist, the mind is passive in registering the qualities of objects; but for Kant the mind actively structures experience into an organized, knowable shape.  Only thus can human knowledge be rescued from skepticism.  Of course, only phenomena are rescued from skepticism; noumena may or may not be always caused or always organized in time and space.  Illusory metaphysics arises when human reason applies its inherent concepts to noumena, to which they do not apply.  Thus attempts to prove the existence of God are futile, for God is never known phenomenally, and so the empirical, if innate, concept of “existence” simply cannot apply to God.  Of course, God cannot be proved not to exist either.

From Plato to Wilhelm Wundt, the aim of psychology was epistemological: how does the human mind arrive  at general Knowledge?  Whatever particular answer was given, empiricist or rationalist, individual differences and the demands of the environment were viewed as mere nuisances.  The focus was clearly on the abstract universal mind as it sought abstract, universal truths.


 Part IV

 The Nineteenth Century

The Enlightenment consensus ended with the French Revolution, which was at first welcomed at the dawn of a greater Age of Reason, but later feared and hated for its Reign of Terror.  The real implications of the geometric spirit became clear, and nineteenth-century thinkers had to come to grips with naturalism.  This task was made more pressing by Darwin’s theory of evolution, which not only made man into an ape but also took all purpose and progress out of history.  The second half of the century was the founding of scientific psychology and the formulation of its three variants: the study of consciousness, of the unconscious, and of adaptation.

We have already met philosophers such as Berkeley, Hume, and Newton who are at least partially positivistic, favoring an epistemology that restricts human knowledge to what is immediately observable.  As an epistemology, positivism adopted a radical empiricism.  Metaphysical speculation and explanations of nature in terms of unobservable entities were to be abandoned.  Instead, human knowledge would confine itself to collecting and correlating facts to yield an accurate description of the world.  Out of the nineteenth century emerged the three founding forms of psychology.  Wundt founded the psychology of consciousness.  Freud found the psychology of the unconscious.  And various evolutionary psychologists founded the psychology of adaptation.

Wilhelm Maximilian Wundt (1832 – 1920)

Although Wilhelm Wundt is revered as the founder of scientific psychology, he is the most misunderstood of all psychology’s major figures.  His system is usually considered to be dualistic, atomistic, associationistic, purely introspective, and concerned only with describing the conscious contents of the normal adult mind viewed as the passive recipient of sense perception.  It was none of these things.  It is, however, often confused with E.B. Titchener’s system which had all these characteristics except dualism.  Titchener was Wundt’s student, but he made Wundt’s voluntaristic psychology into an experimental British associationism, abandoning along the way many Wundtian essentials.  This alteration is one reason for Wundt’s distorted image today.  Another reason is that his psychology was swamped in the later behaviorist movement.  Thus Wundt is remembered primarily as a ponderous old German introspectionist of no importance save as psychology’s founder.







  • ca. 1800 – Franz Joseph Gall developed cranioscopy, the measurement of the skull to determine psychological characteristics, which was later renamed phrenology; it is now discredited.
  • 1807 – Georg Wilhelm Friedrich Hegel published Phenomenology of Spirit (Mind), which describes his thesis-antithesis-synthesis dialectical method, according to which knowledge pushes forwards to greater certainty, and ultimately towards knowledge of the noumenal world.
  • 1808 – Johann Christian Reil coined the term “psychiatry”.










Part V

Twentieth century












Part VI

Scientific Revolutions

Thomas S. Kuhn

The conflict between empiricist and dialectician may be resolved by the work of Thomas S. Kuhn known for his book The Structure of Scientific Revolutions.  He suggest that science has at least two levels or modes of operation: normal science proceeds empirically and incrementally by fitting observations and facts into an agreed body of theories and assumptions (on this level empiricism and positivism however one-dimensional work well); however, periodically sciences undergo revolutions in their underlying paradigms, and these move dialectically so that the new paradigm struggles with and supersedes the old.

By paradigm Kuhn means the patterns of assumption, methods and theories to which scientists make a prior commitment upon joining their professional colleagues.  Paradigms give answers to the following kinds of question.  “What are the fundamental entities of which the universe is composed?  How do these interact with each other and with the senses?  What questions may legitimately be asked about such entities and what techniques employed in seeking solutions.”  The answers supplied by professional education ‘are both rigorous and rigid’ and ‘exert a deep hold on the scientific mind’.  It is necessary when scientists say that they have ‘discovered’ something or ‘tested’ a certain proposition to remember that they are not speaking about their paradigms.  Test, discoveries, measurements, and observations all take place within the paradigm or context.  As Kuhn observes, ‘Normal science and research are strenuous a d devoted attempts to force nature into the conceptual boxes provided by professional education.’

It is no part of the aim of normal science to call forth new sorts of phenomena.  Essentially work within an accepted paradigm consists of puzzle-solving, in a manner reminiscent of jig-saws and crossword puzzles, wherein the rules of solution are given and a combination of high intelligence and convergent thinking is necessary to demonstrate the paradigm’s range and applicability.  To laymen the experiments may seem esoteric and miniscule.  Scientists rarely ask such fundamental questions as whether peace can be assured, cancer cured or crime controlled, for such issues are really within their paradigm’s scope, which accordingly ‘insulates a (scientific) community from the socially important problems that are not reducible to puzzle form’.  So habitual becomes the paradigm that the work ‘metaphysical’ is generally reserved for a priori assumptions which are not one’s own, while questions one does not with to answer are consigned to other disciplines with punishments administered to those straying from the conceptual box.

Kuhn argues from historical evidence that paradigms, unlike hypotheses, are not abandoned when they are falsified, since this would involve relinquishing whole structures of organized knowledge.  Typically, more and more anomalies accumulate until there are a distressing number of phenomena unaccounted for by the paradigm.  It is at this point that rival paradigms arise and the conflicts between them are bitter, ideological, rhetorical and political in style.  Allan Buss, a dialectical psychologist at the University of Calgary, has argued that historically psychology had been subject to cyclic revolutions, from ‘reality constructs the person’ to ‘person constructs reality’.   In applying Kuhn to psychology, Allan Buss argues that both behaviorist and Freudian paradigms assumed that ‘reality construct the person’.  For behaviorist’s, stimuli in the environment and rewards shape the person’s response; for Freudian’s irrational and unconscious forces determine a limited energy mechanism.  In contrast cognitive and humanistic psychologists have stressed that ‘the person constructs reality’ ; while ego psychologists have also stressed mastery.  Historically psychology has oscillated between these four paradigms although all four distort reality.



Existentialism emerged as a protest against the displacement of individual consciousness from the center of life’s stage by a depersonalized nature, a transcendent deity, and/or the collectivized state.  Three centuries  of rapid advance and compartmentalization of sciences, based on Newtonian mechanism, and Cartesian dualism had shattered the human image.  Rival authorities claimed to control the personality.  Existentialism is usually traced to Søren Kierkegaard the Danish philosopher of the early nineteenth century, but is also anticipated in Melville, Dostoevsky and Nietzsche.  But it took the disasters of the early twentieth century, war, depression, Stalin-ism, Fascism before total disenchantment with systems external to the individual engulfed Western cultures.  In a world where Christianity, progress and enlightenment had countenanced Auschwitz it seemed necessary to start again, with the only ideals still untarnished the personal values proclaimed by writers, prisoners and resistance fighters, evoking anguished memories of lost friends and moments of tenderness grasped in the lull between battles.  Anyone who had loved anyone for precious moments had fared better than the corporate worlds of abstractions, of bureaus and machines of absolute ideas and crude messianisms’, the churches bent on institutional survival and collaborating businesses.

So it was that existentialism came to stand for an entire range of missing elements in Western culture.  Where advanced industrialism had stressed the static, the abstract, the objective, the logically rational and unambiguous and the dispassionate universalism of systems detached from the knower, existentialists stressed the dynamic, the concrete, the inter-subjective, consensually validated experiences however ambiguous and passionate uniqueness of the engaged participant.


 Part VII


Mind–body problem

The mind–body problem concerns the explanation of the relationship that exists between minds, or mental processes, and bodily states or processes.  The main aim of philosophers working in this area is to determine the nature of the mind and mental states/processes, and how—or even if—minds are affected by and can affect the body.

Our perceptual experiences depend on stimuli that arrive at our various sensory organs from the external world, and these stimuli cause changes in our mental states, ultimately causing us to feel a sensation, which may be pleasant or unpleasant. Someone’s desire for a slice of pizza, for example, will tend to cause that person to move his or her body in a specific manner and in a specific direction to obtain what he or she wants.  The question, then, is how it can be possible for conscious experiences to arise out of a lump of gray matter endowed with nothing but electrochemical properties.

A related problem is how someone’s propositional attitudes (e.g. beliefs and desires) cause that individual’s neurons to fire and his muscles to contract. These comprise some of the puzzles that have confronted epistemologists and philosophers of mind from at least the time of René Descartes.

Dualist solutions to the mind–body problem

Dualism is a set of views about the relationship between mind and matter (or body).  It begins with the claim that mental phenomena are, in some respects, non-physical.  One of the earliest known formulations of mind–body dualism was expressed in the eastern Sankhya and Yoga schools of Hindu philosophy (c. 650 BCE), which divided the world into purusha (mind/spirit) and prakriti (material substance).  Specifically, the Yoga Sutra of Patanjali presents an analytical approach to the nature of the mind.

In Western Philosophy, the earliest discussions of dualist ideas are in the writings of Plato who maintained that humans’ “intelligence” (a faculty of the mind or soul) could not be identified with, or explained in terms of, their physical body.  However, the best-known version of dualism is due to René Descartes (1641), and holds that the mind is a non-extended, non-physical substance, a “res cogitans“.  Descartes was the first to clearly identify the mind with consciousness and self-awareness, and to distinguish this from the brain, which was the seat of intelligence.  He was therefore the first to formulate the mind–body problem in the form in which it still exists today.

Arguments for dualism

The most frequently used argument in favor of dualism is that it appeals to the common-sense intuition that conscious experience is distinct from inanimate matter. If asked what the mind is, the average person would usually respond by identifying it with their self, their personality, their soul, or some other such entity.  They would almost certainly deny that the mind simply is the brain, or vice versa, finding the idea that there is just one ontological entity at play to be too mechanistic, or simply unintelligible.  Many modern philosophers of mind think that these intuitions are misleading and that we should use our critical faculties, along with empirical evidence from the sciences, to examine these assumptions to determine whether there is any real basis to them.

Another important argument in favor of dualism is that the mental and the physical seem to have quite different, and perhaps irreconcilable, properties.  Mental events have a subjective quality, whereas physical events do not.  So, for example, one can reasonably ask what a burnt finger feels like, or what a blue sky looks like, or what nice music sounds like to a person.  But it is meaningless, or at least odd, to ask what a surge in the uptake of glutamate in the dorsolateral portion of the hippocampus feels like.

Philosophers of mind call the subjective aspects of mental events “qualia” or “raw feels”.  There is something that it is like to feel pain, to see a familiar shade of blue, and so on.  There are qualia involved in these mental events that seem particularly difficult to reduce to anything physical.  David Chalmers explains this argument by stating that we could conceivably know all the objective information about something, such as the brain states and wavelengths of light involved with seeing the color red, but still not know something fundamental about the situation – what it is like to see the color red.

If consciousness (the mind) can exist independently of physical reality (the brain), one must explain how physical memories are created concerning consciousness.  Dualism must therefore explain how consciousness affects physical reality.  One possible explanation is that of a miracle, proposed by Arnold Geulincx and Nicolas Malebranche, where all mind–body interactions require the direct intervention of God.

Another possible argument that has been proposed by C. S. Lewis  is the Argument from Reason: if, as monism implies, all of our thoughts are the effects of physical causes, then we have no reason for assuming that they are also the consequent of a reasonable ground.  Knowledge, however, is apprehended by reasoning from ground to consequent.  Therefore, if monism is correct, there would be no way of knowing this—or anything else—we could not even suppose it, except by a fluke.

The zombie argument is based on a thought experiment proposed by Todd Moody, and developed by David Chalmers in his book The Conscious Mind. The basic idea is that one can imagine one’s body, and therefore conceive the existence of one’s body, without any conscious states being associated with this body.  Chalmers’ argument is that it seems very plausible that such a being could exist because all that is needed is that all and only the things that the physical sciences describe about a zombie must be true of it.  Since none of the concepts involved in these sciences make reference to consciousness or other mental phenomena, and any physical entity can be by definition described scientifically via physics, the move from conceivability to possibility is not such a large one.  Others such as Dennett have argued that the notion of a philosophical zombie is an incoherent,  or unlikely, concept. It has been argued under physicalism that one must either believe that anyone including oneself might be a zombie, or that no one can be a zombie—following from the assertion that one’s own conviction about being (or not being) a zombie is a product of the physical world and is therefore no different from anyone else’s.  This argument has been expressed by Dennett who argues that “Zombies think they are conscious, think they have qualia, think they suffer pains—they are just ‘wrong’ (according to this lamentable tradition) in ways that neither they nor we could ever discover!”  

Interactionist dualism

Interactionist dualism, or simply interactionism, is the particular form of dualism first espoused by Descartes in the Meditations.  In the 20th century, its major defenders have been Karl Popper and John Carew Eccles.  It is the view that mental states, such as beliefs and desires, causally interact with physical states.

Descartes’ famous argument for this position can be summarized as follows: Seth has a clear and distinct idea of his mind as a thinking thing that has no spatial extension (i.e., it cannot be measured in terms of length, weight, height, and so on).  He also has a clear and distinct idea of his body as something that is spatially extended, subject to quantification and not able to think.  It follows that mind and body are not identical because they have radically different properties.

At the same time, however, it is clear that Seth’s mental states (desires, beliefs, etc.) have causal effects on his body and vice versa: A child touches a hot stove (physical event) which causes pain (mental event) and makes her yell (physical event), this in turn provokes a sense of fear and protectiveness in the caregiver (mental event), and so on.

Descartes’ argument crucially depends on the premise that what Seth believes to be “clear and distinct” ideas in his mind are necessarily true. Many contemporary philosophers doubt this.  For example, Joseph Agassi suggests that several scientific discoveries made since the early 20th century have undermined the idea of privileged access to one’s own ideas.  Freud claimed that a psychologically-trained observer can understand a person’s unconscious motivations better than the person himself does.  Duhem has shown that a philosopher of science can know a person’s methods of discovery better than that person herself does, while Malinowski has shown that an anthropologist can know a person’s customs and habits better than the person whose customs and habits they are.  He also asserts that modern psychological experiments that cause people to see things that are not there provide grounds for rejecting Descartes’ argument, because scientists can describe a person’s perceptions better than the person herself can.  The weakness common to all these arguments against interactionism is that they put all introspective insight in doubt.  We know people make mistakes about the world (including another’s internal states), but not always.  Therefore, it is logically absurd to assume persons are always in error about their own mental states and judgements about the nature of the mind itself.

Other forms of dualism

Four varieties of dualism. The arrows indicate the direction of the causal interactions. Occasionalism is not shown.

Psychophysical parallelism

Psychophysical parallelism, or simply parallelism, is the view that mind and body, while having distinct ontological statuses, do not causally influence one another. Instead, they run along parallel paths (mind events causally interact with mind events and brain events causally interact with brain events) and only seem to influence each other.  This view was most prominently defended by Gottfried Leibniz.  Although Leibniz was an ontological monist who believed that only one type of substance, the monad, exists in the universe, and that everything is reducible to it, he nonetheless maintained that there was an important distinction between “the mental” and “the physical” in terms of causation.  He held that God had arranged things in advance so that minds and bodies would be in harmony with each other.  This is known as the doctrine of pre-established harmony.


Occasionalism is the view espoused by Nicholas Malebranche that asserts that all supposedly causal relations between physical events, or between physical and mental events, are not really causal at all. While body and mind are different substances, causes (whether mental or physical) are related to their effects by an act of God’s intervention on each specific occasion.

Property dualism

Property dualism is the view that the world is constituted of just one kind of substance – the physical kind – and there exist two distinct kinds of properties: physical properties and mental properties.  In other words, it is the view that non-physical, mental properties (such as beliefs, desires and emotions) inhere in some physical bodies (at least, brains).  How mental and physical properties relate causally depends on the variety of property dualism in question, and is not always a clear issue.  Sub-varieties of property dualism include:

  1. Strong emergentism asserts that when matter is organized in the appropriate way (i.e. in the way that living human bodies are organized), mental properties emerge in a way not fully accountable for by physical laws. Hence, it is a form of emergent materialism.  These emergent properties have an independent ontological status and cannot be reduced to, or explained in terms of, the physical substrate from which they emerge.  They are dependent on the physical properties from which they emerge, but opinions vary as to the coherence of top–down causation, i.e. the causal effectiveness of such properties.  A form of property dualism has been espoused by David Chalmers and the concept has undergone something of a renaissance in recent years,  but was already suggested in the 19th century by William James.
  2. Epiphenomenalism is a doctrine first formulated by Thomas Henry Huxley.  It consists of the view that mental phenomena are causally ineffectual, where one or more mental states do not have any influence on physical states.  Physical events can cause other physical events and physical events can cause mental events, but mental events cannot cause anything, since they are just causally inert by-products (i.e. epiphenomena) of the physical world.  This view has been defended most strongly in recent times by Frank Jackson.
  3. Non-reductive Physicalism is the view that mental properties form a separate ontological class to physical properties: mental states (such as qualia) are not reducible to physical states.  The ontological stance towards qualia in the case of non-reductive physicalism does not imply that qualia are causally inert; this is what distinguishes it from epiphenomenalism.
  4. Panpsychism is the view that all matter has a mental aspect, or, alternatively, all objects have a unified center of experience or point of view.  Superficially, it seems to be a form of property dualism, since it regards everything as having both mental and physical properties. However, some panpsychists say mechanical behavior is derived from primitive mentality of atoms and molecules—as are sophisticated mentality and organic behavior, the difference being attributed to the presence or absence of complex structure in a compound object.  So long as the reduction of non-mental properties to mental ones is in place, panpsychism is not a (strong) form of property dualism; otherwise it is.

Dual aspect theory

Dual aspect theory or dual-aspect monism is the view that the mental and the physical are two aspects of, or perspectives on, the same substance.  (Thus it is a mixed position, which is monistic in some respects).  In modern philosophical writings, the theory’s relationship to neutral monism has become somewhat ill-defined, but one proffered distinction says that whereas neutral monism allows the context of a given group of neutral elements and the relationships into which they enter to determine whether the group can be thought of as mental, physical, both, or neither, dual-aspect theory suggests that the mental and the physical are manifestations (or aspects) of some underlying substance, entity or process that is itself neither mental nor physical as normally understood.  Various formulations of dual-aspect monism also require the mental and the physical to be complementary, mutually irreducible and perhaps inseparable (though distinct).

Hylomorphic dualism

Monist solutions to the mind–body problem

In contrast to dualism, monism does not accept any fundamental divisions. The fundamentally disseparate nature of reality has been central to forms of eastern philosophies for over two millennia.  In Indian and Chinese philosophy, monism is integral to how experience is understood.  Today, the most common forms of monism in Western philosophy are physicalist.  Physicalistic monism asserts that the only existing substance is physical, in some sense of that term to be clarified by our best science.However, a variety of formulations (see below) are possible.  Another form of monism, idealism, states that the only existing substance is mental.  Although pure idealism, such as that of George Berkeley, is uncommon in contemporary Western philosophy, a more sophisticated variant called panpsychism, according to which mental experience and properties may be at the foundation of physical experience and properties, has been espoused by some philosophers such as Alfred North Whitehead  and David Ray Griffin.

Phenomenalism is the theory that representations (or sense data) of external objects are all that exist.  Such a view was briefly adopted by Bertrand Russell and many of the logical positivists during the early 20th century.  A third possibility is to accept the existence of a basic substance that is neither physical nor mental.  The mental and physical would then both be properties of this neutral substance.  Such a position was adopted by Baruch Spinoza and was popularized by Ernst Mach in the 19th century.  This neutral monism, as it is called, resembles property dualism.

Physicalistic monisms


Behaviorism dominated philosophy of mind for much of the 20th century, especially the first half.   In psychology, behaviorism developed as a reaction to the inadequacies of introspectionism.  Introspective reports on one’s own interior mental life are not subject to careful examination for accuracy and cannot be used to form predictive generalizations.  Without generalizability and the possibility of third-person examination, the behaviorists argued, psychology cannot be scientific.  The way out, therefore, was to eliminate the idea of an interior mental life (and hence an ontologically independent mind) altogether and focus instead on the description of observable behavior.

Parallel to these developments in psychology, a philosophical behaviorism (sometimes called logical behaviorism) was developed.  This is characterized by a strong verificationism, which generally considers unverifiable statements about interior mental life senseless.  For the behaviorist, mental states are not interior states on which one can make introspective reports. They are just descriptions of behavior or dispositions to behave in certain ways, made by third parties to explain and predict another’s behavior.

Philosophical behaviorism has fallen out of favor since the latter half of the 20th century, coinciding with the rise of cognitivism.  Cognitivists reject behaviorism due to several perceived problems.  For example, behaviorism could be said to be counter-intuitive when it maintains that someone is talking about behavior in the event that a person is experiencing a painful headache.

Identity theory

Type physicalism (or type-identity theory) was developed by John Smart and Ullin Place as a direct reaction to the failure of behaviorism.  These philosophers reasoned that, if mental states are something material, but not behavioral, then mental states are probably identical to internal states of the brain.  In very simplified terms: a mental state M is nothing other than brain state B.  The mental state “desire for a cup of coffee” would thus be nothing more than the “firing of certain neurons in certain brain regions”.

The classic Identity theory and Anomalous Monism in contrast.  For the Identity theory, every token instantiation of a single mental type corresponds (as indicated by the arrows) to a physical token of a single physical type.  For anomalous monism, the token–token correspondences can fall outside of the type–type correspondences.  The result is token identity.

Despite its initial plausibility, the identity theory faces a strong challenge in the form of the thesis of multiple realizability, first formulated by Hilary Putnam.  It is obvious that not only humans, but many different species of animals can, for example, experience pain.  However, it seems highly unlikely that all of these diverse organisms with the same pain experience are in the identical brain state.  And if this is the case, then pain cannot be identical to a specific brain state.  The identity theory is thus empirically unfounded.

On the other hand, even granted the above, it does not follow that identity theories of all types must be abandoned.  According to token identity theories, the fact that a certain brain state is connected with only one mental state of a person does not have to mean that there is an absolute correlation between types of mental state and types of brain state.  The type–token distinction can be illustrated by a simple example: the word “green” contains four types of letters (g, r, e, n) with two tokens (occurrences) of the letter e along with one each of the others.  The idea of token identity is that only particular occurrences of mental events are identical with particular occurrences or tokenings of physical events.  Anomalous monism (see below) and most other non-reductive physicalisms are token-identity theories.  Despite these problems, there is a renewed interest in the type identity theory today, primarily due to the influence of Jaegwon Kim.


Functionalism was formulated by Hilary Putnam and Jerry Fodor as a reaction to the inadequacies of the identity theory.  Putnam and Fodor saw mental states in terms of an empirical computational theory of the mind.  At about the same time or slightly after, D.M. Armstrong and David Kellogg Lewis formulated a version of functionalism that analyzed the mental concepts of folk psychology in terms of functional roles.  Finally, Wittgenstein‘s idea of meaning as use led to a version of functionalism as a theory of meaning, further developed by Wilfrid Sellars and Gilbert Harman. Another one, psychofunctionalism, is an approach adopted by the naturalistic philosophy of mind associated with Jerry Fodor and Zenon Pylyshyn.

What all these different varieties of functionalism share in common is the thesis that mental states are characterized by their causal relations with other mental states and with sensory inputs and behavioral outputs.  That is, functionalism abstracts away from the details of the physical implementation of a mental state by characterizing it in terms of non-mental functional properties.  For example, a kidney is characterized scientifically by its functional role in filtering blood and maintaining certain chemical balances.  From this point of view, it does not really matter whether the kidney be made up of organic tissue, plastic nanotubes or silicon chips: it is the role that it plays and its relations to other organs that define it as a kidney.

Non-reductive physicalism

Non-reductionist philosophers hold firmly to two essential convictions with regard to mind–body relations: 1) Physicalism is true and mental states must be physical states, but 2) All reductionist proposals are unsatisfactory: mental states cannot be reduced to behavior, brain states or functional states.  Hence, the question arises whether there can still be a non-reductive physicalism. Donald Davidson‘s anomalous monism is an attempt to formulate such a physicalism.

Davidson uses the thesis of supervenience: mental states supervene on physical states, but are not reducible to them.  “Supervenience” therefore describes a functional dependence: there can be no change in the mental without some change in the physical–causal reducibility between the mental and physical without ontological reducibility.

Because non-reductive physicalist theories attempt to both retain the ontological distinction between mind and body and to try to solve the “surfeit of explanations puzzle” in some way; critics often see this as a paradox and point out the similarities to epiphenomenalism, in that it is the brain that is seen as the root “cause” not the mind, and the mind seems to be rendered inert.

Epiphenomenalism regards one or more mental states as the byproduct of physical brain states, having no influence on physical states.  The interaction is one-way (solving the “surfeit of explanations puzzle”) but leaving us with non-reducible mental states (as a byproduct of brain states) – causally reducible, but ontologically irreducible to physical states.  Pain would be seen by epiphenomenaliasts as being caused by the brain state but as not having effects on other brain states, though it might have effects on other mental states (i.e. cause distress).

Weak emergentism

Weak emergentism is a form of “non-reductive physicalism” that involves a layered view of nature, with the layers arranged in terms of increasing complexity and each corresponding to its own special science.  Some philosophers hold that emergent properties causally interact with more fundamental levels, while others maintain that higher-order properties simply supervene over lower levels without direct causal interaction.  The latter group therefore holds a less strict, or “weaker”, definition of emergentism, which can be rigorously stated as follows: a property P of composite object O is emergent if it is metaphysically impossible for another object to lack property P if that object is composed of parts with intrinsic properties identical to those in O and has those parts in an identical configuration.

Sometimes emergentists use the example of water having a new property when Hydrogen H and Oxygen O combine to form H2O (water).  In this example there “emerges” a new property of a transparent liquid that would not have been predicted by understanding hydrogen and oxygen as gases. This is analogous to physical properties of the brain giving rise to a mental state. Emergentists try to solve the notorious mind–body gap this way.  One problem for emergentism is the idea of “causal closure” in the world that does not allow for a mind-to-body causation.

Eliminative materialism

If one is a materialist and believes that all aspects of our common-sense psychology will find reduction to a mature cognitive neuroscience, and that non-reductive materialism is mistaken, then one can adopt a final, more radical position: eliminative materialism.

There are several varieties of eliminative materialism, but all maintain that our common-sense “folk psychology” badly misrepresents the nature of some aspect of cognition.  Eliminativists such as Patricia and Paul Churchland argue that while folk psychology treats cognition as fundamentally sentence-like, the non-linguistic vector/matrix model of neural network theory or connectionism will prove to be a much more accurate account of how the brain works.

The Churchlands often invoke the fate of other, erroneous popular theories and ontologies that have arisen in the course of history.  For example, Ptolemaic astronomy served to explain and roughly predict the motions of the planets for centuries, but eventually this model of the solar system was eliminated in favor of the Copernican model. The Churchlands believe the same eliminative fate awaits the “sentence-cruncher” model of the mind in which thought and behavior are the result of manipulating sentence-like states called “propositional attitudes“.

Non-physicalist monisms


Idealism is the form of monism that sees the world as consisting of minds, mental contents and or consciousness.  Idealists are not faced with explaining how minds arise from bodies: rather, the world, bodies and objects are regarded as mere appearances held by minds.  However, accounting for the mind–body problem is not usually the main motivation for idealism; rather, idealists tend to be motivated by skepticism, intentionality, and the unique nature of ideas.  Idealism is prominent in Eastern religious and philosophical thought.  It has gone through several cycles of popularity and neglect in the history of Western philosophy.

Different varieties of idealism may hold that there are

Neutral monism

Neutral monism, in philosophy, is the metaphysical view that the mental and the physical are two ways of organizing or describing the same elements, which are themselves “neutral,” that is, neither physical nor mental.  This view denies that the mental and the physical are two fundamentally different things.  Rather, neutral monism claims the universe consists of only one kind of stuff, in the form of neutral elements that are in themselves neither mental nor physical.  These neutral elements might have the properties of color and shape, just as we experience those properties.  But these shaped and colored elements do not exist in a mind (considered as a substantial entity, whether dualistically or physicalistically); they exist on their own.


Some philosophers take an epistemic approach and argue that the mind–body problem is currently unsolvable, and perhaps will always remain unsolvable to human beings.  This is usually termed New mysterianism. Colin McGinn holds that human beings are cognitively closed in regards to their own minds. According to McGinn human minds lack the concept-forming procedures to fully grasp how mental properties such as consciousness arise from their causal basis.  An example would be how an elephant is cognitively closed in regards to particle physics.

A more moderate conception has been expounded by Thomas Nagel, which holds that the mind body problem is currently unsolvable at the present stage of scientific development and that it might take a future scientific paradigm shift or revolution to bridge the explanatory gap.  Nagel posits that in the future a sort of “objective phenomenology” might be able to bridge the gap between subjective conscious experience and its physical basis.

Linguistic criticism of the mind–body problem

Each attempt to answer the mind–body problem encounters substantial problems. Some philosophers argue that this is because there is an underlying conceptual confusion.  These philosophers, such as Ludwig Wittgenstein and his followers in the tradition of linguistic criticism, therefore reject the problem as illusory.  They argue that it is an error to ask how mental and biological states fit together.  Rather it should simply be accepted that human experience can be described in different ways—for instance, in a mental and in a biological vocabulary.  Illusory problems arise if one tries to describe the one in terms of the other’s vocabulary or if the mental vocabulary is used in the wrong contexts.  This is the case, for instance, if one searches for mental states of the brain.  The brain is simply the wrong context for the use of mental vocabulary—the search for mental states of the brain is therefore a category error or a sort of fallacy of reasoning.

Today, such a position is often adopted by interpreters of Wittgenstein such as Peter Hacker.  However, Hilary Putnam, the originator of functionalism, has also adopted the position that the mind–body problem is an illusory problem which should be dissolved according to the manner of Wittgenstein.

Externalism and internalism

Where is the mind located?  If the mind is a physical phenomenon of some kind, it has to be located somewhere.  According to some, there are two possible options: either the mind is internal to the body (internalism) or the mind is external to it (externalism).  More generally, either the mind depends only on events and properties taking place inside the subject’s body or it depends also on factors external to it.

Proponents of internalism are committed to the view that neural activity is sufficient to produce the mind.  Proponents of externalism maintain that the surrounding world is in some sense constitutive of the mind.

Externalism differentiates into several versions.  The main ones are semantic externalism, cognitive externalism, phenomenal externalism.  Each of these versions of externalism can further be divided whether they refer only to the content or to the vehicles of mind.

Semantic externalism holds that the semantic content of the mind is totally or partially defined by state of affairs external to the body of the subject. Hilary Putnam‘s Twin earth thought experiment is a good example.

Cognitive externalism is a very broad collection of views that suggests the role of the environment, of tools, of development, and of the body in fleshing out cognition.  Embodied cognition, the extended mind, and enactivism are good examples.

Phenomenal externalism suggests that the phenomenal aspects of the mind are external to the body.  Authors who addressed this possibility are Ted Honderich, Edwin Holt, Francois Tonneau, Kevin O’Regan, Riccardo Manzotti, Teed Rockwell and Max Velmans.

Naturalism and its problems

The thesis of physicalism is that the mind is part of the material (or physical) world.  Such a position faces the problem that the mind has certain properties that no other material thing seems to possess.  Physicalism must therefore explain how it is possible that these properties can nonetheless emerge from a material thing.  The project of providing such an explanation is often referred to as the “naturalization of the mental”.  Some of the crucial problems that this project attempts to resolve include the existence of qualia and the nature of intentionality.


Many mental states seem to be experienced subjectively in different ways by different individuals.  And it is characteristic of a mental state that it has some experiential quality, e.g. of pain, that it hurts.  However, the sensation of pain between two individuals may not be identical, since no one has a perfect way to measure how much something hurts or of describing exactly how it feels to hurt.  Philosophers and scientists therefore ask where these experiences come from.  The existence of cerebral events, in and of themselves, cannot explain why they are accompanied by these corresponding qualitative experiences.  The puzzle of why many cerebral processes occur with an accompanying experiential aspect in consciousness seems impossible to explain.

Yet it also seems to many that science will eventually have to explain such experiences.  This follows from an assumption about the possibility of reductive explanations.  According to this view, if an attempt can be successfully made to explain a phenomenon reductively (e.g., water), then it can be explained why the phenomenon has all of its properties (e.g., fluidity, transparency).  In the case of mental states, this means that there needs to be an explanation of why they have the property of being experienced in a certain way.

The 20th century German philosopher Martin Heidegger criticized the ontological assumptions underpinning such a reductive model, and claimed that it was impossible to make sense of experience in these terms.  This is because, according to Heidegger, the nature of our subjective experience and its qualities is impossible to understand in terms of Cartesian “substances” that bear “properties”.  Another way to put this is that the very concept of qualitative experience is incoherent in terms of—or is semantically incommensurable with the concept of—substances that bear properties.

This problem of explaining introspective first-person aspects of mental states and consciousness in general in terms of third-person quantitative neuroscience is called the explanatory gap.  There are several different views of the nature of this gap among contemporary philosophers of mind. David Chalmers and the early Frank Jackson interpret the gap as ontological in nature; that is, they maintain that qualia can never be explained by science because physicalism is false.  There are two separate categories involved and one cannot be reduced to the other.  An alternative view is taken by philosophers such as Thomas Nagel and Colin McGinn.  According to them, the gap is epistemological in nature.  For Nagel, science is not yet able to explain subjective experience because it has not yet arrived at the level or kind of knowledge that is required.  We are not even able to formulate the problem coherently.  For McGinn, on other hand, the problem is one of permanent and inherent biological limitations.  We are not able to resolve the explanatory gap because the realm of subjective experiences is cognitively closed to us in the same manner that quantum physics is cognitively closed to elephants.  Other philosophers liquidate the gap as purely a semantic problem.  This semantic problem, of course, led to the famous “Qualia Question“, which is: Does Red cause Redness?


John Searle—one of the most influential philosophers of mind, proponent of biological naturalism (Berkeley 2002)

Intentionality is the capacity of mental states to be directed towards (about) or be in relation with something in the external world.  This property of mental states entails that they have contents and semantic referents and can therefore be assigned truth values.  When one tries to reduce these states to natural processes there arises a problem: natural processes are not true or false, they simply happen.  It would not make any sense to say that a natural process is true or false.  But mental ideas or judgments are true or false, so how then can mental states (ideas or judgments) be natural processes?  The possibility of assigning semantic value to ideas must mean that such ideas are about facts.  Thus, for example, the idea that Herodotus was a historian refers to Herodotus and to the fact that he was a historian.  If the fact is true, then the idea is true; otherwise, it is false.  But where does this relation come from?  In the brain, there are only electrochemical processes and these seem not to have anything to do with Herodotus.

Philosophy of perception

Philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual objects, in particular how perceptual experience relates to appearances and beliefs about the world.  The main contemporary views within philosophy of perception include naive realism, enactivism and representional views.

Philosophy of mind and science

Humans are corporeal beings and, as such, they are subject to examination and description by the natural sciences.  Since mental processes are intimately related to bodily processes, the descriptions that the natural sciences furnish of human beings play an important role in the philosophy of mind.  There are many scientific disciplines that study processes related to the mental.  The list of such sciences includes: biology, computer science, cognitive science, cybernetics, linguistics, medicine, pharmacology, and psychology.


The theoretical background of biology, as is the case with modern natural sciences in general, is fundamentally materialistic.  The objects of study are, in the first place, physical processes, which are considered to be the foundations of mental activity and behavior.  The increasing success of biology in the explanation of mental phenomena can be seen by the absence of any empirical refutation of its fundamental presupposition: “there can be no change in the mental states of a person without a change in brain states.”

Within the field of neurobiology, there are many subdisciplines that are concerned with the relations between mental and physical states and processes:  Sensory neurophysiology investigates the relation between the processes of perception and stimulationCognitive neuroscience studies the correlations between mental processes and neural processes. Neuropsychology describes the dependence of mental faculties on specific anatomical regions of the brain.  Lastly, evolutionary biology studies the origins and development of the human nervous system and, in as much as this is the basis of the mind, also describes the ontogenetic and phylogenetic development of mental phenomena beginning from their most primitive stages.  Evolutionary biology furthermore places tight constraints on any philosophical theory of the mind, as the gene-based mechanism of natural selection does not allow any giant leaps in the development of neural complexity or neural software but only incremental steps over long time periods.

Since the 1980s, sophisticated neuroimaging procedures, such as fMRI (above), have furnished increasing knowledge about the workings of the human brain, shedding light on ancient philosophical problems.

The methodological breakthroughs of the neurosciences, in particular the introduction of high-tech neuroimaging procedures, has propelled scientists toward the elaboration of increasingly ambitious research programs: one of the main goals is to describe and comprehend the neural processes which correspond to mental functions (see: neural correlate).  Several groups are inspired by these advances.

Computer science

Computer science concerns itself with the automatic processing of information (or at least with physical systems of symbols to which information is assigned) by means of such things as computers.  From the beginning, computer programmers have been able to develop programs that permit computers to carry out tasks for which organic beings need a mind.  A simple example is multiplication.  But it is clear that computers do not use a mind to multiply.  Could they, someday, come to have what we call a mind? This question has been propelled into the forefront of much philosophical debate because of investigations in the field of artificial intelligence (AI).

Within AI, it is common to distinguish between a modest research program and a more ambitious one: this distinction was coined by John Searle in terms of a weak AI and strong AI.  The exclusive objective of “weak AI”, according to Searle, is the successful simulation of mental states, with no attempt to make computers become conscious or aware, etc.  The objective of strong AI, on the contrary, is a computer with consciousness similar to that of human beings.  The program of strong AI goes back to one of the pioneers of computation Alan Turing.  As an answer to the question “Can computers think?”, he formulated the famous Turing test.  Turing believed that a computer could be said to “think” when, if placed in a room by itself next to another room that contained a human being and with the same questions being asked of both the computer and the human being by a third party human being, the computer’s responses turned out to be indistinguishable from those of the human.  Essentially, Turing’s view of machine intelligence followed the behaviourist model of the mind—intelligence is as intelligence does.  The Turing test has received many criticisms, among which the most famous is probably the Chinese room thought experiment formulated by Searle.

The question about the possible sensitivity (qualia) of computers or robots still remains open.  Some computer scientists believe that the specialty of AI can still make new contributions to the resolution of the “mind body problem”.  They suggest that based on the reciprocal influences between software and hardware that takes place in all computers, it is possible that someday theories can be discovered that help us to understand the reciprocal influences between the human mind and the brain (wetware).


Psychology is the science that investigates mental states directly.  It uses generally empirical methods to investigate concrete mental states like joy, fear or obsessions.  Psychology investigates the laws that bind these mental states to each other or with inputs and outputs to the human organism.

An example of this is the psychology of perception.  Scientists working in this field have discovered general principles of the perception of forms.  A law of the psychology of forms says that objects that move in the same direction are perceived as related to each other.  This law describes a relation between visual input and mental perceptual states.  However, it does not suggest anything about the nature of perceptual states.  The laws discovered by psychology are compatible with all the answers to the mind–body problem already described.

Cognitive science

Cognitive science is the interdisciplinary scientific study of the mind and its processes.  It examines what cognition is, what it does, and how it works.  It includes research on intelligence and behavior, especially focusing on how information is represented, processed, and transformed (in faculties such as perception, language, memory, reasoning, and emotion) within nervous systems (human or other animal) and machines (e.g. computers).  Cognitive science consists of multiple research disciplines, including psychology, artificial intelligence, philosophy, neuroscience, linguistics, anthropology, sociology, and education.  It spans many levels of analysis, from low-level learning and decision mechanisms to high-level logic and planning; from neural circuitry to modular brain organisation.  Rowlands argues that cognition is enactive, embodied, embedded, affective and (potentially) extended.  The position is taken that the “classical sandwich” of cognition sandwiched between perception and action is artificial; cognition has to be seen as a product of a strongly coupled interaction that cannot be divided this way.

Philosophy of mind in the continental tradition

Most of the discussion in this article has focused on one style or tradition of philosophy in modern Western culture, usually called analytic philosophy (sometimes described as Anglo-American philosophy).  Many other schools of thought exist, however, which are sometimes subsumed under the broad (and vague) label of continental philosophy.  In any case, though topics and methods here are numerous, in relation to the philosophy of mind the various schools that fall under this label (phenomenology, existentialism, etc.) can globally be seen to differ from the analytic school in that they focus less on language and logical analysis alone but also take in other forms of understanding human existence and experience.  With reference specifically to the discussion of the mind, this tends to translate into attempts to grasp the concepts of thought and perceptual experience in some sense that does not merely involve the analysis of linguistic forms.

Immanuel Kant’s Critique of Pure Reason, first published in 1781 and presented again with major revisions in 1787, represents a significant intervention into what will later become known as the philosophy of mind. Kant’s first critique is generally recognized as among the most significant works of modern philosophy in the West.  Kant is a figure whose influence is marked in both continental and analytic/Anglo-American philosophy.  Kant’s work develops an in-depth study of transcendental consciousness, or the life of the mind as conceived through universal categories of consciousness.

In Georg Wilhelm Friedrich Hegel‘s Philosophy of Mind (frequently translated as the Philosophy of Spirit or Geist), the third part of his Encyclopedia of the Philosophical Sciences, Hegel discusses three distinct types of mind: the “subjective mind/spirit”, the mind of an individual; the “objective mind/spirit”, the mind of society and of the State; and the “Absolute mind/spirit”, the position of religion, art, and philosophy.  See also Hegel’s Phenomenology of Spirit. Nonetheless, Hegel’s work differs radically from the style of Anglo-American philosophy of mind.

In 1896, Henri Bergson made in Matter and Memory “Essay on the relation of body and spirit” a forceful case for the ontological difference of body and mind by reducing the problem to the more definite one of memory, thus allowing for a solution built on the empirical test case of aphasia.

In modern times, the two main schools that have developed in response or opposition to this Hegelian tradition are phenomenology and existentialism. Phenomenology, founded by Edmund Husserl, focuses on the contents of the human mind (see noema) and how processes shape our experiences.  Existentialism, a school of thought founded upon the work of Søren Kierkegaard, focuses on Human predicament and how people deal the situation of being alive.  Existential-phenomenology represents a major branch of continental philosophy (they are not contradictory), rooted in the work of Husserl but expressed in its fullest forms in the work of Martin Heidegger, Jean-Paul Sartre, Simone de Beauvoir and Maurice Merleau-Ponty. See Heidegger’s Being and Time, Merleau-Ponty’s Phenomenology of Perception, Sartre’s Being and Nothingness, and Simone de Beauvoir’s The Second Sex.

Mind in Eastern philosophy

Hindu philosophy


Substance Dualism is a common feature of several orthodox Hindu schools including the Sāṅkhya, Nyāya, Yoga and Dvaita Vedanta.  In these schools a clear difference is drawn between matter and a non-material soul, which is eternal and undergoes samsara, a cycle of death and rebirth.  The Nyāya school argued that qualities such as cognition and desire are inherent qualities which are not possessed by anything solely material, and therefore by process of elimination must belong to a non-material self, the atman.  Many of these schools see their spiritual goal as moksha, liberation from the cycle of reincarnation.

Vedanta monistic idealism


In the Advaita Vedanta of the 8th century Indian philosopher Śaṅkara, the mind, body and world are all held to be the same unchanging eternal conscious entity called Brahman.  Advaita, which means non-dualism, holds the view that all that exists is pure absolute consciousness.  The fact that the world seems to be made up of changing entities is an illusion, or Maya.  The only thing that exists is Brahman, which is described as Satchitananda (Being, consciousness and bliss).  Advaita Vedanta is best described by a verse which states “Brahman is alone True, and this world of plurality is an error; the individual self is not different from Brahman”.

Another form of monistic Vedanta is Vishishtadvaita (Qualified Non-Dualism) as posited by the eleventh century philosopher Ramanuja.  Ramanuja criticized Advaita Vedanta by arguing that consciousness is always intentional and that it is also always a property of something.  Ramanuja’s Brahman is defined by a multiplicity of qualities and properties in a single monistic entity.  This doctrine is called “samanadhikaranya” (several things in a common substrate).


Arguably the first exposition of empirical materialism in the history of philosophy is in the Cārvāka school (a.k.a. Lokāyata).  The Cārvāka school rejected the existence of anything but matter (which they defined as being made up of the four elements), including God and the soul.  Therefore they held that even consciousness was nothing but a construct made up of atoms. A section of the Cārvāka school believed in a material soul made up of air or breath, but since this also was a form of matter, it was not said to survive death.

Buddhist philosophy of mind

A salient feature of Buddhist philosophy which sets it apart from Indian orthodoxy is the centrality of the doctrine of not-self (Pāli. anatta, Skt. anātman).  The Buddha’s not-self doctrine sees humans as an impermanent composite of five psychological and physical aspects instead of a single fixed self.  In this sense, what is called ego or the self is merely a convenient fiction, an illusion that does not apply to anything real but to an erroneous way of looking at the ever changing stream of five interconnected aggregate factors.  The relationship between these aggregates is said to be one of dependent-arising (pratītyasamutpāda).  This means that all things, including mental events, arise co-dependently from a plurality of other causes and conditions.  This seems to reject both causal determinist and epiphenomenalist conceptions of mind.  Abhidharma theories of mind three centuries after the death of the Buddha (c. 150 BCE) saw the growth of a large body of literature called the Abhidharma in several contending Buddhist schools.  In the Abdhidharmic analysis of mind, the ordinary thought is defined as prapañca (‘conceptual proliferation’).  According to this theory, perceptual experience is bound up in multiple conceptualizations (expectations, judgments and desires).  This proliferation of conceptualizations form our illusory superimposition of concepts like self and other upon an ever changing stream of aggregate phenomena.  In this conception of mind no strict distinction is made between the conscious faculty and the actual sense perception of various phenomena. Consciousness is instead said to be divided into six sense modalities, five for the five senses and sixth for perception of mental phenomena.  The arising of cognitive awareness is said to depend on sense perception, awareness of the mental faculty itself which is termed mental or ‘introspective awareness’ (manovijñāna) and attention (āvartana), the picking out of objects out of the constantly changing stream of sensory impressions.  Rejection of a permanent agent eventually led to the philosophical problems of the seeming continuity of mind and also of explaining how rebirth and karma continue to be relevant doctrines without an eternal mind.  This challenge was met by the Theravāda school by introducing the concept of mind as a factor of existence.  This “life-stream” (Bhavanga-sota) is an undercurrent forming the condition of being.  The continuity of a karmic “person” is therefore assured in the form of a mindstream (citta-santana), a series of flowing mental moments arising from the subliminal life-continuum mind (Bhavanga-citta), mental content, and attention.  Indian MahayanaThe Sautrāntika school held a form of phenomenalism that saw the world as imperceptible.  It held that external objects exist only as a support for cognition, which can only apprehend mental representations.  This influenced the later Yogacara school of Mahayana Buddhism.  The Yogācāra school is often called the mind-only school because of its internalist stance that consciousness is the ultimate existing reality.  The works of Vasubandhu have often been interpreted as arguing for some form of IdealismVasubandhu uses the dream argument and a mereological refutation of atomism to attack the reality of external objects as anything other than mental entities.  Scholarly interpretations of Vasubandhu‘s philosophy vary widely, and include phenomenalism, neutral monism and realist phenomenology.  The Indian Mahayana schools were divided on the issue of the possibility of reflexive awareness (svasaṃvedana).  Dharmakīrti accepted the idea of reflexive awareness as expounded by the Yogacara school, comparing it to lamp that illuminates itself while also illuminating other objects.  This was strictly rejected by Mādhyamika scholars like Candrakīrti.  Since in the philosophy of the Mādhyamika all things and mental events are characterized by emptiness, they argued that consciousness could not be an inherently reflexive ultimate reality since that would mean it was self validating and therefore not characterized by emptiness.  These views were ultimately reconciled by the 8th century thinker Śāntarakṣita.  In Śāntarakṣita‘s synthesis he adopts the idealist Yogācāra views of reflexive awareness as a conventional truth into the structure of the two truths doctrine.  Thus he states: “By relying on the Mind-Only system, know that external entities do not exist.  And by relying on this Middle Way system, know that no self exists at all, even in that [mind].”  The Yogācāra school also developed the theory of the repository consciousness (ālayavijñāna) to explain continuity of mind in rebirth and accumulation of karma.  This repository consciousness acts as a storehouse for karmic seeds (bija) when all other senses are absent during the process of death and rebirth as well as being the causal potentiality of dharmic phenomena.  Thus according to B. Alan Wallace:

No constituents of the body—in the brain or elsewhere—transform into mental states and processes. Such

subjective experiences do not emerge from the body, but neither do they emerge from nothing.  Rather, all objective mental appearances arise from the substrate, and all subjective mental states and processes arise from the

substrate consciousness

Tibetan Buddhism

Tibetan Buddhist theories of mind evolved directly from the Indian Mahayana views.  Thus the founder of the Gelug school, Je Tsongkhapa discusses the Yogācāra system of the Eight Consciousnesses in his Explanation of the Difficult Points.  He would later come to repudiate Śāntarakṣita‘s pragmatic idealism.  According to the 14th Dalai Lama the mind can be defined “as an entity that has the nature of mere experience, that is, ‘clarity and knowing’.  It is the knowing nature, or agency, that is called mind, and this is non-material.”  The simultaneously dual nature of mind is as follows:

1. Clarity (gsal) – The mental activity which produces cognitive phenomena (snang-ba).
2. Knowing (rig) – The mental activity of perceiving cognitive phenomena.

Because Tibetan philosophy of mind is ultimately soteriological, it focuses on meditative practices such as Dzogchen and Mahamudra that allow a practitioner to experience the true reflexive nature of their mind directly. This unobstructed knowledge of one’s primordial, empty and non-dual Buddha nature is called rigpa.  The mind’s innermost nature is described among various schools as pure luminosity or “clear light” (‘od gsal) and is often compared to a crystal ball or a mirror.  Sogyal Rinpoche speaks of mind thus: “Imagine a sky, empty, spacious, and pure from the beginning; its essence is like this.  Imagine a sun, luminous, clear, unobstructed, and spontaneously present; its nature is like this.”

Zen Buddhism

The central issue in Chinese Zen philosophy of mind is in the difference between the pure and awakened mind and the defiled mind.  Chinese Chan master Huangpo described the mind as without beginning and without form or limit while the defiled mind was that which was obscured by attachment to form and concepts.  The pure Buddha-mind is thus able to see things “as they truly are”, as absolute and non-dual “thusness” (Tathatā).  This non-conceptual seeing also includes the paradoxical fact that there is no difference between a defiled and a pure mind, as well as no difference between samsara and nirvana.  In the Shobogenzo, the Japanese philosopher Dogen argued that body and mind are neither ontologically nor phenomenologically distinct but are characterized by a oneness called shin jin (bodymind).  According to Dogen, “casting off body and mind” (Shinjin datsuraku) in zazen will allow one to experience things-as-they-are (genjokoan) which is the nature of original enlightenment (hongaku).

There are countless subjects that are affected by the ideas developed in the philosophy of mind.  Clear examples of this are the nature of death and its definitive character, the nature of emotion, of perception and of memory. Questions about what a person is and what his or her identity consists of also have much to do with the philosophy of mind.  There are two subjects that, in connection with the philosophy of the mind, have aroused special attention: free will and the self.

Free will

In the context of philosophy of mind, the problem of free will takes on renewed intensity.  This is certainly the case, at least, for materialistic determinists.  According to this position, natural laws completely determine the course of the material world.  Mental states, and therefore the will as well, would be material states, which means human behavior and decisions would be completely determined by natural laws.  Some take this reasoning a step further: people cannot determine by themselves what they want and what they do.  Consequently, they are not free.  This argumentation is rejected, on the one hand, by the compatibilists.  Those who adopt this position suggest that the question “Are we free?” can only be answered once we have determined what the term “free” means.  The opposite of “free” is not “caused” but “compelled” or “coerced”.  It is not appropriate to identify freedom with indetermination.  A free act is one where the agent could have done otherwise if it had chosen otherwise.  In this sense a person can be free even though determinism is true.  The most important compatibilist in the history of the philosophy was David Hume.  More recently, this position is defended, for example, by Daniel Dennett.  On the other hand, there are also many incompatibilists who reject the argument because they believe that the will is free in a stronger sense called libertarianism.  These philosophers affirm the course of the world is either a) not completely determined by natural law where natural law is intercepted by physically independent agency,  b) determined by indeterministic natural law only, or c) determined by indeterministic natural law in line with the subjective effort of physically non-reducible agency.  Under Libertarianism, the will does not have to be deterministic and, therefore, it is potentially free.  Critics of the second proposition (b) accuse the incompatibilists of using an incoherent concept of freedom.  They argue as follows: if our will is not determined by anything, then we desire what we desire by pure chance.  And if what we desire is purely accidental, we are not free.  So if our will is not determined by anything, we are not free.


The philosophy of mind also has important consequences for the concept of self.  If by “self” or “I” one refers to an essential, immutable nucleus of the person, most modern philosophers of mind will affirm that no such thing exists.  The idea of a self as an immutable essential nucleus derives from the idea of an immaterial soul.  Such an idea is unacceptable to most contemporary philosophers, due to their physicalistic orientations, and due to a general acceptance among philosophers of the scepticism of the concept of “self” by David Hume, who could never catch himself not doing, thinking or feeling anything.  However, in the light of empirical results from developmental psychology, developmental biology and neuroscience, the idea of an essential inconstant, material nucleus—an integrated representational system distributed over changing patterns of synaptic connections—seems reasonable.  The view of the self as an illusion is accepted by some philosophers, including Daniel Dennett.


What is Consciousness?  {choose your conception here}