Philosophy of Mind

The main precursor of the modern concept of mind is the ancient Greek notion of soul (psyche), which was originally used to mark the difference between things that are alive and things that are dead. Conceptually, it was related to breath (pneuma), and was thought to come in degrees corresponding to different states of consciousness. Thus, a dead man was said to have lost his psyche entirely, whereas a sleeping or fainting man has lost enough of it to lose consciousness, though that too would bring him a step closer to death. The soul is composed of extremely light and tenuous matter, variously identified with pure and "breathable" elements such as air (as by the philosophers Thales [c. 625–c. 547 B.C.E.] and Anaximenes [570?–500? B.C.E.]) or fire (by Heraclitus [c. 540–480 B.C.E.]). Most pre-Socratic thinkers would have understood the expression "he breathed his last" literally, and seen the dying gasps of a Homeric warrior, say, as the exhalation of his soul. If psyche could exist in a disembodied state—and it is doubtful whether most early Greek philosophers thought that it could—it would have been as a shadowy or ephemeral form, like the denizens of Hades.

With Plato (c. 428–348 or 347 B.C.E.), philosophers began to ask more sophisticated questions about how one can feel, think, possess knowledge, and choose rightly. Plato's strategy here was to divide the soul in terms of its capacities, producing the first faculty psychology in the western tradition.

In his earlier writings, Plato came closer than any other ancient or medieval philosopher to advancing a dualist account like that of Descartes. In the Phaedo, he took immortality to be an essential feature of the soul, so that "mortal soul" is a contradiction in terms. Thus, "when death comes to a man, the mortal part of him dies, but the immortal part retires at the approach of death and escapes unharmed and indestructible" (106e). But a soul immortal by definition has more in common with what is cosmic and divine than with anything in the visible realm—such as its own body, for example—which is why Plato shows little interest in exploring its everyday operations. The soul's union with the body is not its natural state; in fact, the whole point of philosophy is to prepare the soul for its release from the "prison" of the body (80c–84b).

Plato is more forthcoming in his later works, where the concept of the soul plays a major role in his explanation of moral conflict and human action. From the fact that one can be affected by two or more desires simultaneously, he infers that the soul cannot be unitary, since it is impossible for the same thing to act in opposite ways at the same time (there are obvious affinities here with the logical principle of non-contradiction, which Plato learned from Socrates [c. 470–399 B.C.E.]). Accordingly, in the Republic he identifies three distinct parts of the soul—reason (nous), passion (thumos), and appetite (epithumia) —and posits these as the source of conflicting desires (IV, 439d–e). Reason rules over the soul with wisdom, but opposed to it is appetite, the irrational part of the soul "with which it loves, hungers, thirsts, and feels the flutter and titillation of other desires" (439d). Reason and appetite would remain in unending combat but for the intervention of passion, the "spirited" part of the soul that helps reason subdue appetite. Plato has in mind here the experience of steeling our resolve, when we angrily force ourselves to do something we don't want to do because reason has judged it to be the best course of action. What is important about this model, however, is the elevation of rationality to the dominant position in the soul, and conversely, the denigration of appetite as an irrational force that threatens to destroy our well-being. In the Phaedrus, Plato likens reason to a charioteer trying to control two horses: a good horse (passion), who "needs no whip" because he is driven by the command of reason alone, and a bad horse (appetite), who is hard to control and who would run the chariot into the ditch if left unchecked (253d–254e). Plato saw no redeeming value for the emotions in human moral life, though anger, at least, could sometimes be placed in the service of reason.

Aristotle (384–322 B.C.E.) took the study of the soul in a less speculative direction. He wrote an entire treatise on the soul—Peri psyche, or De anima in its Latin translation—that views the soul as a natural phenomenon. The soul is defined as "an actuality of the first kind of a natural organized body" (412b4): that which makes the body alive and capable of performing its characteristic functions. Aristotle divides these into vegetative powers, concerned with nutrition and growth; sensory powers (that is, vision, hearing, taste, smell, and touch, as well as the internal senses of imagination and memory); and intellectual powers (understanding, assertion, and discursive thinking). In his broader taxonomy of life forms, these correspond to the souls of plants, brute animals, and human beings, respectively, with the higher forms subsuming the powers of those lower in the hierarchy (so brute animals are capable of nutrition and sensation but not understanding, whereas human beings exhibit all three capacities). Only the third capacity is relevant to the modern conception of mind, though it should be noted that this is far from being a dualist conception. Aristotle's intellect is a perfection of the organic unity of body and soul, and although he concedes that the activity of thinking is "separable, impassible, and unmixed" (430a17), thinking cannot occur without sensory images.

The Hellenistic period was a time of great philosophical activity, but unfortunately, most of the primary sources have been lost. Thankfully, scholars have managed to piece together some of what was said from fragments of texts and reports in the work of other philosophers. Three figures and their representative schools were especially important. Epicurus (341–270 B.C.E.) revitalized the materialist doctrine of earlier atomists such as Democritus (470–360 B.C.E.), arguing that the soul is composed of extremely light and mobile atoms (thereby accounting for the spontaneity of sensation), whose unpredictable "swerves" are the source of human free choice. Particular features of living things result from the blending of soul atoms, of which there are four different types, each with its own psychological effect: fiery (bodily heat), air-like (rest), wind-like (movement), and a fourth, nameless atom responsible for sensation. The soul is also quickly dispersed at death, its atoms moving on to rain invisibly through the cosmos until joining together with other atoms to make new things. This led Epicurus to argue that "that most frightful of evils, death, is nothing to us, seeing that when we exist death is not present, and when death is present we do not exist" (Letter to Menoeceus, 124-25; Long and Sedley, 24A). Death is literally nothing to an Epicurean, though whether this is sufficient to dispel the fear of death remains an open question.

The Stoics were another Hellenistic school with a materialistic conception of the soul, though their theory was highly nuanced and more teleologically sophisticated than even Aristotle's. Rejecting the tripartite accounts of Plato and Aristotle, they held that the soul is unitary. As Chrysippus of Soli (c. 280–206 B.C.E.), one of the leading Stoics, explained, the soul is a mixture of air and fire (pneuma) whose "parts flow from their seat in the heart, as if from the source of a spring, and spread through the whole body. They continually fill all the limbs with vital breath, and rule and control them with countless different powers—nutrition, growth, locomotion, sensation, impulse to action" (Calcidius, Commentary on Plato's Timaeus, 220; Long and Sedley, 53G). Because their pneuma exists in a certain tension, animal souls can register sense impressions and initiate impulses (hormē), or move towards what they desire. Humans are higher still because they are capable of assenting to (or rejecting) these impressions. Assent is just rational impulse, "a movement of thought towards something in the sphere of action" (Stobaeus, 2.86, 17–87, 6; Long and Sedley, 53Q). Thus, words are sounds arising through our vocal apparatus from impulses in the heart, so that on the Stoic view, "language is sent out imprinted, and stamped as it were, by the conceptions present in thought" (Galen, On Hippocrates' and Plato's Doctrines, 2.5, 9–13; Long and Sedley, 53U).

Plato's teachings were refined throughout the Hellenistic period, culminating in the movement that later came to be known as Neoplatonism. Its foremost practitioner was Plotinus (205–270 C.E.), who offered an original synthesis of the Platonic and Aristotelian perspectives, advancing the former as the model of intelligible reality and the latter of sensible reality. There are three principles in his metaphysical system: the One, the utterly transcendent and unknowable source of everything in the universe; Intelligence or Mind (nous), which is where all eternal and necessary truths are actively thought; and Soul, which is the discursive manifestation of Mind's activity in lesser beings throughout the cosmos. Intellectual thinking is a quasi-mystical act through which we move beyond ourselves and our particular bodily circumstances to grasp eternal principles: "We are not separated from the One, not distant from it, even though bodily nature has closed about us and drawn us to itself" (Enneads, VI, 9.ix). From the Stoics, Plotinus borrows the concept of "seminal reasons," or patterns implanted in Soul by Mind, which Soul then uses to produce the sensible world. This idea was later picked up by Christian thinkers such as St. Augustine of Hippo (354–430), who explains providence in terms of seminal reasons existing eternally in the mind of God.