[Note: The article was intended for inclusion in A. Weber, (Ed.) (in press). Psychology. Vol. 1: History of Psychology. Danbury, CT: Grolier International. Grollier finally decided to publish a greatly abridged version, so I have decided to post the original full-length version here. -cdg-]

Ancient Greek Psychology

Christopher D. Green
York University

Toronto, Canada

The idea of the mind was first systematically explored in ancient Greece.The main figures in this exploration were the early Presocratic philosophers, Plato, Artistotle, and the Hippocratic physicians. The key issues for them were the basic nature of the mind (i.e., what it is made of), and the various parts or functions it has. They also made some early discoveries about the relation between the mind and the brain.

Key Dates Box
Thales (fl. ca. 585 bc)
Anaximander (ca. 611-ca. 545 bc)
Anaximenes (fl. ca.550 bc)
Pythagoras (ca. 570-ca. 500 bc)
Heraclitus (ca. 540-ca. 480 bc)
Parmenides (ca. 515-ca. 445 bc)
Zeno of Elea (b. ca. 490 bc)
Melissus of Samos (fl. ca. 440 bc)
Empedocles of Acragas (ca. 492-432 bc)
Democritus of Abdera (b. ca. 460 bc)
Anaxagoras of Clazomenae (ca. 500-428 bc)
Gorgias of Leontini, ca. 490-ca. 390 bc
Socrates (ca. 470-399 bc)
Plato (ca. 428-ca. 348 bc)
Aristotle (384-322 bc)
Hippocrates of Cos (ca.460-370 bc)
Praxagoras of Cos (fl. ca. 300 bc)
Herophilus of Chalcedon (fl. ca. 270 bc)
Erasistratus of Ceos (fl. ca. 260 bc)

Key Works Box

McKirihan, R. D. Philosophy before Socrates. Hackett, 1994. (Translation and interpretation of all Presocratic fragments)

Plato. Republic. (many translations available)

Plato. Timaeus. (many translations available)

Aristotle. De Anima. (many translations available)

Hippocratic Corpus (see Hippocratic Writings. ed. G.E.R. Lloyd. Penguin, 1983)

Key Points Box

- Presocratics searched mainly for the underlying nature of the psychÍ (i.e., of what is it made?).

- Plato divided the psychÍ into three parts, corresponding to roughly to reason, emotion, and desire, and believed the part corresponding to reason to be immortal.

- Aristotle believed the psychÍ to actualize the body's potential for life, but also to be inseparable form the body. Divided the psychÍ into at least five faculties.

- Hippocratics focused mainly on mental illness, which was thought often to be caused by an imbalance of humours; made neurological discoveries and developed idea of pneuma into an elaborate biopsychological theory.

1. Presocratic Philosophers

Philosophy, as the Western world uses the term, began in Ancient Greece. The very first philosophers lived Miletus, a city in the region of Ionia on the west coast of present-day Turkey (see map). We know fairly little about their ideas, but we do know that they thought about the nature of world and of what they called the psychÍ (which was only loosely related to what we think of as the "mind" today).

The first of these early Ionian philosophers was Thales (fl. ca. 585 bc). We know only three of his claims: (1) all is water, (2) the universe has psychÍ and is full of gods, and (3) magnets have psychÍ. Why he thought everything to be made of water is a mystery, though we know that one of the key philosophical projects of the time was to find the one basic element that underlay all others. Thales chose water.His other two comments mention psychÍ, and seem to show us that the main feature attributed to psychÍ at this time was the ability to cause things to move.Since many aspects of the universe move -- the stars and planets overhead, the ocean tides in and out, the wind, even occasionally the earth underfoot itself -- Thales concluded that the universe must have its own psychÍ.Magnets, as we know, can cause pieces of certain kinds of metals to move, and so they were thought to have psychÍ as well.

Thales' immediate followers at Miletus were Anaximander (ca. 611-ca. 545 bc) and Anaximenes (fl. mid-6th c. bc).It seems that both wanted, like Thales, to discover the basic element of the world, and why the universe moved on its own. Anaximander rejected Thales account that water was the basic element, saying instead that there had once been something he called "the unbounded" (to apeiron, in Greek) out of which all the other elements were extracted by the action of the spinning universe. Anaximenes, by contrast, believed the basic element to have been air, probably because air had long been associated with psychÍ (which in very ancient times seems to have referred simply to breath).If the original element were air, this might have explained to Anaximenes why the universe had psychÍ, and thus could move on its own.

In 546 bc the Persians took control of Ionia, and many of the Greeks living there were forced to emigrate elsewhere.One such emigrant was Pythagoras (ca. 570-ca. 500 bc) who was born and raised on Samos, an island off the coast of Ionia. He is said to have traveled widely (possibly to Egypt and Babylonia, though these reports are doubted by many scholars today), and he finally established a school in Croton, a city in the southern part of present-day Italy.Because he did not write anything (that we know of) and his school was very secretive, we know little of what Pythagoras actually taught. His school continued long after him, and always attributed their own findings to "Pythagoras," so many ideas are connected with his name were probably not known to Pythagoras himself (such as the "Pythagorean theorem" in geometry).We do know that his school was fascinated by numbers, and thought them, rather than say water or air, to be the basic "building blocks" of the universe.The Pythagoreans believed the psychÍ to be a ratio of two numbers. We are told that Pythagoras stressed the study of mathematics as a means of purifying the psychÍ. Through the study of mathematics, it was claimed, the psychÍ is returned, however briefly, to the harmonies thought to be inherent in the cosmos, liberated from its imprisonment in the body. This may have been the earliest claim that the psychÍ had cognitive powers (powers of thought) rather than just being an initiator of movement. The Pythagoreans also believed the psychÍ to be immortal, passing from one body to the next -- some human, some animal -- in an endless series of lives, a doctrine called "metempsychosis."

Although many Greeks moved from Ionia after the Persian invasion, at least one major philosopher, Heraclitus (ca. 540-ca. 480 bc), continued living in his home city of Ephesus, on the coast of Ionia about 50 km from Miletus. He despised Pythagoras, calling him "the chief captain of swindlers," and an "artful knave."It is often said that Herlaclitus claimed everything to be continually in flux, but this appears to miss his main point, which was that the matter out of which a thing is composed does not alone define its existence. There is a constant underlying structure or organization that determines the thing's essence. For instance, Heraclitus is often mistakenly quoted as having said that "it is not possible to step twice into the same river." What he seems to have actually said, however, was "as they step into the same rivers, different and still different waters flow." The river remains the same but its material composition changes as different water flows through it. Heraclitus was stressing unity amidst change, rather than simply change itself. This idea of an underlying structure governing the organization of the cosmos was termed logos, which is often translated as "law," "account," or "word").

Heraclitus had many things to say about the psychÍ. First, wetness is bad for the psychÍ, leading most to believe that Heraclitus believed it to be made of fire: "For psychÍs it is death to become water," he once said. Second, the psychÍ is a mysterious object that one can never know fully: "One would never discover the limits of psychÍ, should one traverse every road, so deep a logos does it possess." Third, psychÍ and strong emotion are seen as opposites: "It is difficult to fight thumos, for whatever it wishes it buys at the price of psychÍ." Thumos is another important psychological term used by the Greeks, who thought it to be responsible for anger, indignation, courage, and other action-oriented emotional states. It is often translated as "spirit." Heraclitus also believed the psychÍ to be immortal, at least in noble people.

After Heraclitus, philosophical thinking turned toward more abstract questions such as are there many things in the world or just one?, do things move or are they forever still?, and are things in distinct locations in space or are they one and the same place. This change in focus came about because Parmenides (ca. 515-ca. 445 bc) from the city of Elea (on the west coast of present-day Italy) and his students-- particularlyZeno of Elea (b. ca. 490 bc) and Melissus of Samos (fl. ca. 440 bc) -- produced powerful arguments against the common sense views in all these cases. Important counterarguments came from Empedocles of Acragas (ca. 492-432 bc), Democritus of Abdera (b. ca. 460 bc), and Anaxagoras of Clazomenae (ca. 500-428 bc). The details of this debate need not detain us here, but these last three also developed accounts of the nature of the mind as well.

Empedocles may have been trained as a Pythagorean, but broke away. He believed in metempsychosis, like the Pythagoreans. He also proposed an influential theory of perception. He claimed that all material object continually give off "effluences" -- tiny copies of themselves -- in all directions, and that the sense organs can pick these up and transmit them to the heart where they become known to the individual.Democritus believed that all things are made of tiny indivisible atoms.The psychÍ too was thought to be composed of atoms. These "psychic" atoms were believed to be the smallest and smoothest of all the atoms, which would explain the rapidity of perception and thought. Anaxagoras was born on the Ionian coast, but spent most of his adult life in Athens. He did not have much to say about psychÍ, but developed a theory about another term that was important to the Greek view of the mind, NŰus.NŰus is a kind of intellect or rationality. In the ordinary Greek of the era, nŰus referred to the ability to think, but Anaxagoras transformed it to mean the rationality of the cosmos as a whole -- the principle that keeps the world orderly and moving according to plan.

2. Socrates and Plato

Over the course of Anaxagoras' life, the city of Athens became the cultural and economic center of the Greek world.Traveling teachers, known as "sophists" (from the Greek word for wisdom) came to Athens to teach the city's youth everything from math to literature to politics.Because Athens was a democracy at this time, learning to speak well in public had become a precious commodity, and the some of the most successful sophists (most notably Gorgias of Leontini, ca. 490-ca. 390 bc) held rhetoric -- the art of persuasive speaking -- in higher esteem than truth itself.Athens had also embarked upon a long war with Sparta that it would ultimately lose, bringing down both Athens' power and, for a time, its democratic form of government.

Even during the Athens' darkest days, there was one man who would roam the marketplace, engaging people in debate about the nature of virtue, of truth, of justice, and of the good, usually showing that they were not as knowledgeable about such things as they believed themselves to be.This man was named Socrates (ca. 470-399 bc), and he was a native of Athens. A small group of young men gathered about him in order to learn, even though he said that he knew nothing himself. In 399 bc Socrates was put on public trial for corrupting the youth and teaching of false gods, and was ultimately convicted and put to death.

Some of the Socrates' group of young followers wrote down the debates in which they had heard him engage. One of these followers, in particular, would become a great philosopher in his own right and would ultimately develop his own theories of the true, the good, etc. This man was Plato (ca. 428-ca. 348 bc), also a native of Athens. Plato's writings are far-ranging, and arguably the most influential philosophical work in the history of Western thought.Over the course of his career, he developed an elaborate theory of the psychÍ unlike anything that had gone before. In order to understand it fully, however, one must know a little about his theory of knowledge.

a. Plato's Theory of Knowledge.

How do we come to know things? How do we know, for instance, that a man is a man, and that horse is a horse?Plato believed that in order to be able to identify all men as men, there must be something that ties them together.This something isn't just their similarity in appearance, because men look many different ways -- some are short and some are tall; some dark and some light.Plato hypothesized that there must be, somewhere, an Idea of Man which all men "participate" in or "reflect" in some way.The same for horses -- there must be an Idea of Horse in which all horses participate. The same again for more abstract ideas such as virtue and justice -- all individual just acts are just simply because they reflect the Idea of Justice. These Ideas are, today, more commonly called Platonic "Forms."

We cannot find these Forms by looking around, according to Plato.When we look at an individual man, for instance, we see only an imperfect, pale reflection of the true Form of Man.Plato set out the following allegory (Republic 514a-518b):Imagine some prisoners chained into a sitting position in a cave, facing a wall.A fire is lit outside the cave, and between the fire and prisoners statues of humans and animals are paraded back and forth, casting shadows on the cave wall in front of the prisoners.Because the prisoners have never seen anything else, they believe the shadows to be reality.If one were freed, however, and turned around to look out of the cave, at first s/he would be dazzled by the light of the fire, but as s/he approached the statues s/he would come to see that they were the real things, and that shadows were mere images of them.Similarly, Plato argued, the things around us in the world are like distorted shadows of the Forms.In order to come to know the Forms, we cannot depend on just looking "straight ahead," as it were.They cannot be found that way. Instead, we have to explore further by thinking about the essence of each thing -- the essence of what it is to be a man, to be a horse, or to be just, or good.For Plato, thinking about the Forms is the core of what it is to be a philosopher; it is analogous to breaking our chains, walking out of the cave, and encountering reality directly.

One of Plato's students, quite possibly Aristotle (384-322 bc), discovered a flaw in the theory of Forms (in the Parmenides).Forms were supposed to explain what ties different instances of the same thing -- different men, for instance -- together into a single category.Joe is a man and Fred is man because they both participate in the Form of Man.But what ties the individual instance to the Form?That is, what ties Joe to the Form of Man?There must be another Form -- a "Third Man" -- to do the job. But if so, there must be yet another Form to tie the "Third Man" to the original Form of Man as well, and so on into an infinite regress of Forms.There is little explicit discussion of the theory of Forms in Plato's later works.To this day, historians of philosophy debate whether Plato repudiated the theory of Forms after hearing the Third Man argument.


b. Plato's Theory of the PsychÍ.

Plato's theory changed and developed over the course of his life. In his early works, he wrote of one's ability to improve one's psychÍ through learning (Apology), and of the psychÍ being the source of human morality (Crito). In the Protagoras the psychÍ is called "something which you value more highly than your body," and in the Gorgias the psychÍ is said to be "in command" of the body. In the Meno, he asserted that the psychÍ is the seat of all knowledge and that knowledge is not learned through experience, but is instead inborn. Experience only brings this inborn knowledge to consciousness, through a process of recollection (anamnesis). In the Phaedo, Plato says that the psychÍ is immortal and one's own bodily death will, in fact, constitute a freeing of the psychÍ to be with the Forms that one can only glimpse, via philosophy, while one's psychÍ is trapped in the body.

In the larger works of Plato's middle period, the theory of the psychÍ became more complex. In the Republic, Plato argued that the psychÍ is made of three distinct parts: the intellect which he called the logistikon (literally, the apprehender of the logos), the "spirited" or emotional part forwhich he used the traditional term, thumos (or thumoeides), and the seat of appetites and desires which he called the epithumetikon.

These three parts make up what is often called Plato's tripartite (i.e., three-part) theory of the psychÍ. In the best psychÍ, Plato argued, the logistikon rules, harmonizing the needs of the various parts through the use of reason. People in whom the logistikon rules their psychÍs, he argued, should be the rulers of society because they are not distracted by momentary perceptions or desires, but look to the eternal truths of the Forms for guidance.In the poorer psychÍ, by contrast, some other part rules. For instance, if the epithumetikon takes over, the person will be dominated by appetites and desires. Plato, however, recognized that such people could still serve a useful function in society -- for instance, by generating wealth as businesspeople -- as long as they were ultimately answerable to the laws of the logistikon-dominated rulers.

Although the distinction between reason and desire is well understood, it is more difficult to get a clear sense of the thumos. In at least some instances it seems to map reasonably well on to what would later be called the will. It is the "action" part of the psychÍ; the part that turns plans into results. As noted above, however, the thumos was also thought to be responsible for anger, courage, indignation, and the like. Thus Plato argued that people whose psychÍs are dominated by the thumos would make good soldiers.

In the late-period dialogue, Timaeus, Plato presenteda quasi-religious story about how human beings, including their psychÍs, were constructed by a cosmic "Demiurge" (a Greek term meaning "craftsman").He also gave an extended account of the senses, and how he believed perception to work.The eyes, he said, continually give off a "visual stream" of "pure fire" which make vision possible.The different sensations of touch are caused, according to Plato, by the different geometric shapes of the various kinds of things. Earth is hard because it is formed of cubes that have "wide bases" and so resist our touch. Fire is made of tetrahedrons (i.e., triangular pyramids) that are sharp, and so causes pain when touched. Air is made up of octahedrons (eight-sided figures), and water of icosahedrons (20-sided figures). Having so many sides, they easily slip around each other and out of our grasp when we touch them. Plato also presented in the Timaeus his ideas about human physiology and illness, including mental illness. He saw these as being caused by a bodily imbalances and by poor upbringing and training.Finally, in his very latest works, such the Laws, there was another change in Plato's view of the psychÍ. It seems he lost faith in the idea that the psychÍ is divided into three parts, reverting to discussing it as having two parts --- intellect and desire.


3. Aristotle

Many of the students at Plato's school, called the Academy, went on to become successful philosophers in their own right.None, however, would ultimately have as much influence on Western culture as did Aristotle (384-322 bc).Aristotle was born in the Macedonia, the son of king's physician.In 367 he was sent to Athens to be educated, where he spent 20 years.After living for a few years in northern Ionian, he was called back to Macedonia in 342 bc to tutor young Prince Alexander, who would grow up to be Alexander the Great.In 336 Aristotle returned to Athens to found his own school, the Lyceum.

a. Aristotle's Definition of PsychÍ

Aristotle wrote the first book specifically on the topic of the psychÍ of which we know.In Greek it was called Peri PsychÍs, but today it usually goes by its Latin title, De Anima, or the English translation of this phrase, On the Soul. The term "soul," however, carries religious connotations for us today that it did not have for Aristotle, and so should be used with caution here.The book is really about what life is, and about what relationship holds between the psychÍ and the body such that the two can combine to produce a living being.

Aristotle worked from a series of analogies. First, he compared the relation between psychÍ and body to the relation between a finished house and the pile of bricks of which it was made. He also compared the relation between body and psychÍ to that between a lump of wax and a pattern stamped into it. In a third analogy, Asitotle said, "if some tool, say an axe, were a natural body, its substance would be being-an-axe, and this would then be its psychÍ." Finally, in a fourth analogy he said, "if the eye was an animal, then sight would be its psychÍÖ. "So just as pupil and sight are the eye, so, in our case, psychÍ and body are the animal."

What was Aristotle trying to get at with these four analogies? Consider first analogy of the wax.The impression in the wax is a form (i.e., a pattern of organization) that has been taken on by some matter (namely, the wax).Aristotle's believed, generally speaking, that all things could be analyzed as a certain kind of matter (hylÍ) given a certain form (morphÍ). (This metaphysical position of Aristotle's is called "hylomorphism," derived from the two Greek terms.) He believed further that the approach can be used to analyze the relation of the psychÍ to the body; the psychÍ is the form (or, perhaps better, "organization") given to the matter of the body. His argument goes as follows.

(1) Substances are made of matter, form, or a combination of matter and form.
(2) Bodies are material substances.
(3) Some bodies have life.
(4) The psychÍ, which endows bodies with life, therefore, could not also be material (such as air, fire, etc.) because there would then be two kind of matter in the same place at the same time, a logical absurdity.
(5) Thus, the psychÍ must be a form. Together the body, as matter, and the psychÍ, as form, produce a new substance: the living thing.

But what kind of thing would the psychÍ have to be in order to be the "form of life"? Bodies must be organized in particular ways in order to be alive. If the various parts come "unglued," so to speak, life ends. In addition to a particular anatomical organization, they must continuously operate in certain ways -- taking in nourishment, circulating blood, eliminating toxins, etc. -- fulfilling the needs of the body's various parts. The psychÍ might be thought of, then, as just the living body's organization and operation. Without these a body is just a corpse; it does not have the "form" (in Aristotle's sense) necessary for life. Aristotle thus said that the material body has potentiality or capacity (dynamis) for life. The psychÍ, being form, gives actuality (energeia) to this potential, in the sense that it makes body actually alive -- together they make the living thing. Indeed, Aristotle thought it generally true that matter is merely the potential to be something, and that this potential is actualized by combining it with a particular form.For instance, a lump of stone is potentially a statue of Aristotle, but this potential is only actualized if the stone is given Aristotle's form.

In the other three metaphors Aristotle developed and extended this analysis. A house, for instance, combines the matter of bricks with certain structural principles to ensure that the house is sturdy, weatherproof, etc. These structural principles serve as the form of the house to be sure, but note that they are also closely related to the house's function -- viz., that of being a shelter. A function, such as that of being a shelter, is defined by certain goals or aims, such as keeping one warm and dry. Functions, then, are what Aristotle called final causes, goals for the sake of which a thing is the way it is. Aristotle believed that final causes and forms (also called "formal causes") are often interrelated in this way. The metaphors of the eye and the axe emphasize this functional aspect. Sight is clearly the function (goal, final cause) of the eye. "Being an axe" -- that is, having the capacity to chop -- is the function of an axe. Note that in both cases, the capacity to carry out their functions is closely related to their forms: deform an eye and it cannot see properly; blunt the edge of an axe and it cannot chop properly.

Exactly how we are to understand the claim that the psychÍ is the final cause of life is not entirely clear. Aristotle attempted to clarify it with his concept of "entelechy," a term that refers to a state of completion or perfection. The psychÍ, he said, is "the first entelechy of a natural body that potentially has life."Attempting to elucidate this idea further, he continued that "the entelechy of each thing is naturally inherent in its potentiality, that is in its own matter." Unfortunately, the meaning of this is still not terribly clear. Aristotle seems to have been saying that, by enabling the body to live, the psychÍ actualizes and completes the body's potential for being a living thing. It makes actual the body's "purpose" of being alive, and thus acts as the final cause of life.

b. Aristotle's Faculties of the PsychÍ

Aristotle also described in De Anima the various functions or faculties of the psychÍ.He is far from consistent on the matter, however. At one point, he gave the following definition: "We say that a thing is alive if, for instance, there is intellect or perception or spatial movement and rest or indeed movement connected with nourishment and growth and decay." At another he wrote that "...the psychÍ comprises cognition, perception, and belief. It also comprises appetite, wishing and desire in general. It is the source of locomotion for animals, and also of growth, flourishing, and decay." At still another, he said that the psychÍ has five faculties: nutritive, perceptive, disderative (i.e., that of desire), locomotive, and intellective (nŰus).In addition, he often brought up the question of imagination, but it is not clear if he intended it to be a distinct faculty, or just a way of using the perceptive faculty (see below).

Whatever number of faculties Aristotle's believed the psychÍ to have, it is clear that he believed them to be arranged in a ladder or hierarchy. The psychÍs of the simple life-forms have only the most basic faculties. Those of the more complex life-forms, have the more sophisticated faculties in addition to the basic ones. Plants, for instance, "go on living as long as they are able to take nourishment. This faculty can be separated from the others but the others cannot be separated from this in mortal things." That is, all living things have the nutritive faculty. Although some have the nutritive faculty without having perception and the rest, none have perception without also having the nutritive faculty. Nutrition is basic to life. It is the only faculty plants have.

In order to be an animal, according to Aristotle, a living thing must have the faculty of perception as well. This is the second rung on Aristotle's hierarchy of faculties. Perception, according to Aristotle, is the reception of the shape of an object, without its matter. Here he invokes again the analogy of a form being pressed into a wax block by another object: the wax takes the shape of the thing pressed into it, but not the thing itself. Similarly, he says, sight is the taking of the visible form of an object without taking the object itself.Within the perceptive faculty, Aristotle built a subordinate hierarchy of the various senses. Touch, he says, is the most basic. Taste, is second, because of its relation to nutrition. The order of the rest is not spelled out explicitly, but we can surmise them to be smell, then hearing, then sight. He also proposed a "common sense" in which the various kinds of sensation are combined into a single, integrated mental image.

In discussing perception, Aristotle also commented that if an animal has perception then it must also have imagination and desire. Imagination, he says, seems to be a kind of movement in the sense-organs. Perception itself was thought to be such a movement, but one caused by things outside the organism. Imagination, on the other hand, is a sort of voluntary movement of the sense organs, causing them to respond as though they were perceiving.

Locomotion is the next highest rung on the ladder, above nutrition and perception. Aristotle attributed it only to some animals. Finally, a few animals are said to have, in addition, "the thinking faculty and intellect, such as man and any other creature there may be like him or superior to him." Laying out the exact nature of the intellect was a difficult problem for Aristotle. Heclaimed that in order to have the capacity to understand things apart from itself, the intellectual faculty must have no material properties of its own, for these would interfere with its ability to know. He later explained this by saying that "the intellect is in a way potentially the objects of thought, but nothing in actuality before it thinks, and the potentiality is like that of the tablet on which there is nothing actually written." This is the famed tabula rasa, or "blank slate," metaphor of the mind. The mind must have nothing on "written on it" in advance, so the analogy goes, for knowledge to be "written on it" freely by experience. Aristotle also distinguished between what he called "active" and "passive" parts of the intellect, a distinction that has caused no end of debate. Aristotle gave the active intellect little specific function, other than to say that it is superior to the passive intellect because of its activity. He also said that it is "immortal and eternal," which would seem to contradict his earlier claim that "the psychÍ is not separable from the animal." So perplexing is this conflict that some have contended that the passage on the active intellect must have been added afterwards by someone else.

4. The Greek Medical Tradition

In addition to the philosophical tradition, there was in ancient Greece a distinct and sometimes opposed tradition of learning -- the medical tradition -- that had much to say about the nature of the mind.Perhaps the best known representative of this tradition was Hippocrates of Cos (ca.460-370 bc). The medical writings known as the Hippocratic Corpus were not all written by Hippocrates, but they were written by members of the medical tradition he began.These writing are marked by preference for observation over abstract reason (though some of the observations reported are clearly inaccurate), and a preference for concrete explanations (involving, for instance, climate, diet, or bodily fluids) over metaphyscial ones.

In many of the writings, health was said to be related to a balance among competing or opposing elements (e.g., hot/cold, dry/wet).In particular, maintaining the correct balance among the four bodily fluids known as the humours -- blood, phlegm, yellow bile, and black bile -- was thought to be crucial to health. Disease was thought to be caused by an imbalance among these. Each of the humours was also related to one of the four basic "elements":


Imbalances among these were thought to cause specific mental problems.The terms can still be found in our modern vocabulary: Too much blood would cause one to be "sanguine," too much phlegm "phlegmatic," too much yellow bile "choler" (anger), and too much black bile "melancholy").

Apart from the Hippocratice Oath to which physicians swear even today, the Hippocratic corpus is perhaps most famous for the treatise entitled "The Sacred Disease," in which it is claimed that the condition now known as epilepsy does not have divine causes, as was widely believed at the time, but rather is caused by the brain.There was a competing school of thought within the medical tradition, however, that believed the heart, rather than the brain, to be the main source of thought and behavior.†† One of the leaders of this competing school of thought was Praxagoras of Cos (fl. ca. 300 bc) who discovered the difference between arteries and veins (though he sometimes confused these with what we now know to be nerves).Because he worked only on corpses, however, he found the arteries to be empty, while the veins contained blood. He reasoned that the arteries must carry, in life, some substance other than blood, and concluded that this substance must be something called pneuma -- a term closely associated with "breath" in ancient Greek, and so with life itself.Pneuma was thought to be, like breath, a kind of hot air.Naturally, when the body died, it was thought to lose its pneuma.

During the 3rd century bc, a small group of surgeons in Alexandria, Egypt -- most notably Herophilus of Chalcedon (fl. ca. 270 bc) and Erasistratus of Ceos (fl. ca. 260 bc) -- were given permission by the local ruler to vivisect criminals in order to study the workings of the living body.Herophilus and Eriststratus were able to clearly distinguish the artery and vein systems from the nervous system, and establish that the nervous system was centered on the brain rather than the heart.The were even able to distinguish between sensory (receptor) and motor (effector) nerve pathways. They maintained, however, Praxagoras' belief that arteries carry pneuma from the heart to the rest of the body. Indeed Erasistratus elaborated on the theory, arguing that the arteries distribute a "vital pneuma" responsible for basic biological functions such as metabolism, and that the nerves distribute what he called "psychic pneuma" which was thought to be responsible for mental function.