For nearly two millennia, Aristotle's De Anima (On the Soul) exercised a profound influence on Western philosophy. In his impressive work, Aristotle seeks to uncover the principle of life-whatever it is that separates the living from the dead. In this, the first systematic treatment of human psychology, Aristotle tries to steer a course between two extremes. On one side lies a thoroughgoing materialism, which sees all of reality, including the human mind, as ultimately composed of nothing more than atoms in the void. On the other is the dualism of his teacher Plato, which sees the mind as a separate entity that can exist independently of the body. Aristotle invents a third kind of position that takes mental phenomena to be thoroughly dependent on, though not reducible to, physical events. In this respect, his work resonates with many contemporary developments in philosophy of mind.

One of the most influential philosophers ever to have lived, Aristotle was born in the northern Greek town of Stageira in 384 BCE. When he was no more than seventeen years old, Aristotle made his way to Athens, where he soon became a member of Plato's Academy, an informal group that met to discuss philosophy with the master. He remained in the Academy for twenty years, until Plato's death in 348, when he left Athens and ultimately turned up as the tutor of the future Macedonian king, Alexander. When Alexander took the throne, Aristotle returned to Athens to found his own school, the Lyceum. Nearing the end of his life, Aristotle fled Athens in 323; this may have been a consequence of the anti-Macedonian sentiment Alexander's imperialism inspired in the Athenians. According to some (dubious) reports, Aristotle said he "would not let Athens sin twice against philosophy," a reference to the fate that city had meted out to Socrates. He died a year later in Chalcis, having retreated to his mother's family's estates.

To grapple with this difficult but rewarding text, we need to know something about the context in which Aristotle was writing, and about the history of his corpus itself. While scholars are largely in agreement about the authenticity of those works now attributed to Aristotle, it is extremely unlikely that Aristotle himself ever intended them to be made public. The organization of his works into discrete books and even chapters is probably the work of a first century BCE figure, Andronicus. The contemporary academic myth that Aristotle's texts were originally lecture notes (Aristotle was not doing anything that resembled college teaching today) probably has a grain of truth in it. The prose is often enigmatic and abbreviated, which suggests that its author was not writing for an audience but rather jotting down ideas he would later verbally expound.

All of this means that reading Aristotle can be an invigorating mental exercise, or an exercise in frustration, depending on one's temperament. The reader has to do considerably more work making sense of Aristotle than, say, Plato. More than this, however, we have to beware of distorting Aristotle's thought by seeing it only through the lens of contemporary concepts. Ancient authors allow us to conceive the world as they did, revealing our way of carving up the conceptual territory as but one among many.

Nowhere is this clearer than in De Anima. It is surprising to open a book so titled and find a treatment of the nutrition and growth of plants. But "soul" or psuchê does not mean for Aristotle what it means for us: his concern is with the principles of life, and "soul" designates whatever it is that sets the living off from the nonliving. He thus examines life in all its forms. The human soul will simply be whatever animating principle explains the distinctive powers of human beings, just as a plant's soul explains its growth and development. Anyone purporting to be interested in souls today, or indeed any time after Descartes, would take himself to be investigating the nature of the mind and its thoughts, emotions, and so on, in contrast to the purely physical events of the brain. But this is not the interesting contrast for Aristotle. He wants to discover the difference between the animate and inanimate, and this inquiry necessarily extends to all of the natural world, not just human beings.

For Aristotle, every living thing, from the lowliest fungus to the hairless ape, has a soul. This does not mean, of course, that every living thing has some element in it that will survive its death, as the Pythagoreans thought. Indeed, if we leave to one side for the moment his problematic discussion of the intellect, Aristotle is quite clear: no soul can survive the death of the body it animates.

To see why this is so, we must know something about Aristotle's larger philosophical system, which provides the framework for his discussion of life. The crucial terms here are "form" and "matter." For Aristotle, every ordinary "middle-sized dry good," in the canonical phrase, is a substance, or a "this," and is composed of both form and matter. The matter (hule) is the stuff a thing is made of; the form (morphe) is its organizing principle or structure; hence Aristotle's position is known as "hylomorphism." In his Physics, Aristotle introduces these notions with the example of a statue: the bronze statue of Posiedon has bronze as its matter, and a certain shape as its form. Its shape is what makes it what it is, viz., a statue of Posiedon, as opposed to, say, Pan; its matter is what makes it this particular statue of Poseidon and not some other. There is obviously no way to separate this particular shape (or Poseidon-ness) from the bronze that serves as its matter; conversely, there is no way this statue can exist as a statue without having some matter or other. This is a tricky point, and a crucial one for understanding Aristotle's views on mind. For it is not necessary that the statue have exactly the parts of bronze that it does (it can be repaired, with new bronze added, and still continue to exist). What counts is that any form must be the form of some matter or other. Plato, by contrast, had thought that the forms, the features of things that make them the kinds of things they are, existed in a separate realm altogether, with physical things only "participating" in them. Aristotle thinks this makes no sense: there is no way for a shape or any form generally to exist without being enmattered.

We can see the difference between form and matter as a difference between two kinds of explanation. One way to explain the growth of a plant would be in purely material terms: this bit of stuff moved over here, and then more stuff followed suit, etc. But any such explanation is irremediably incomplete: we have to make reference to the kind of thing the plant is to explain why the matter is organized in the way it is. Aristotle clearly has a point here. If all inquiry had to be conducted in terms of the fundamental particles of physics, there would be no such thing as biology, for example: we wouldn't be able to appeal to the fact that this seed is an apple rather than a pear seed in explaining why the resulting tree develops as it does. Nor would there be any such thing as psychology, as we understand it: we would have to talk only in terms of brain states, and even then this would only be shorthand for talk about protons and electrons, or bosons and quarks.

This background lets us put Aristotle's criticisms of his predecessors in book 1 in their proper context. Materialists like Democritus cannot be right, for they will not have explained all that needs explaining; in particular, they have no account of what makes one body a living thing and another a mere collection of parts. Nor can they explain why animate beings, from paramecia to dogs, develop as they do. But neither can dualists like Plato be right, for they take the form of a thing to be separable from its matter. What is more important, they don't pay enough attention to the role of matter in constituting natural bodies: "the supporters of such theories merely undertake to explain the nature of the soul. Of the body which is to receive it they have nothing more to say: just as if it were possible for any soul taken at random. . . to pass into any body" (I. iii).

Nevertheless, we must avoid thinking of form and matter as separate ingredients that together make up a substance. The form is not an extra element over and above the matter; as we have seen, form and matter never exist on their own. Rather, matter is always matter structured by some form, just as form is always the form of this or that bit of matter. In the context of the souls or animating principles of living things, Aristotle writes, "If, then, we have to make a general statement touching the soul in all its forms, the soul will be the first actuality of a natural body furnished with organs" (II. i). That is, the soul is the principle that makes a naturally occurring body the kind of body it is. "Hence there is no need to enquire whether soul and body are one, any more than whether the wax and the imprint are one, or, in general, whether the matter of a thing is the same with that of which it is the matter" (ibid).

The importance of all of this, especially for contemporary philosophers, lies in Aristotle's clear recognition of distinct ways of approaching natural bodies without positing an extra substance that mysteriously animates them. In the case of plants, this is fairly clear; they have what Aristotle calls a "nutritive soul," that is, a principle of life that allows them to take in nutrition and to grow. (All living things have this; non-human animals also have a "sensitive soul" that explains their ability to perceive their environment, while humans have both kinds of soul plus a "rational soul" that endows them with the power of reason.) The true study of living things will concern both the form and the matter (I. iii).

In precisely the same way, we can understand anger in purely physical terms, in terms of the heat surrounding the heart; but we have not fully understood anger until we invoke its formal elements, for example, a disposition to hurt the object of one's anger (I. i). The form of a living thing, its soul, just is the set of capacities and dispositions the thing has. From this point of view, the soul is not a thing at all; "to speak of the soul as feeling anger is as if one should say that the soul weaves or builds" (I. iv). It is the person, the form/matter compound, who feels anger, not his soul, just as it is the dog, and not the form of the dog, that digests food.

Unfortunately, this relatively simple picture is complicated by Aristotle's puzzling remarks on the possibility of certain parts of the soul surviving the death of the body. For the operations of the intellect, unlike the emotions, do not depend on the body, and hence there is a chance for the operations of the intellect to continue after death. At the same time, Aristotle insists that no individual thing, such as a person, can be eternal, or persist forever as one and the same thing (II. iv). This is one reason why animals reproduce; they have a share in the divine and the eternal by continuing to exist, not as the same person or dog they are, but by producing an admittedly imperfect copy of themselves.

Although it is the most difficult aspect of Aristotle's position to grasp, his insistence on the separability of some faculties of the intellect provided the basis for his continuing influence throughout the Middle Ages, right up until the seventeenth century, for it seemingly made his view consistent with Christianity. After mostly disappearing for several centuries, Aristotle's texts resurfaced, thanks largely to Islamic commentators, and they became the centerpiece of medieval philosophy. And in the thirteenth century, Aristotelian thought reached its apogee in Summa Theologica, the monumental work of St. Thomas Aquinas, who knitted Aristotelianism and Christianity into a nearly seamless whole. For precisely the reasons we have been examining, Aquinas does not conceive of the afterlife as the survival of the soul: "I am not my soul, and if only souls are saved, neither I nor any human is saved." If the afterlife simply consisted of the intellect existing without a body, it would hardly be worth wanting, since it is not the whole person who survives, but merely some of his capacities. The separability of these capacities, however, is crucial in explaining what happens to the soul in the awkward interval between the death of the body and its resurrection.

Aristotle's dominance crumbled in the seventeenth century, with the definitive advances of the scientific revolution. The mechanical philosophy invented by such thinkers as Descartes and Galileo sought to explain all physical phenomena in terms of the push and pull of bodies on one another; the varied phenomena of nature do not require Aristotelian forms but can instead be explained simply by the variations in the microstructural features of the bodies we see around us. In particular, plant and animal life in all its forms depends merely on the ways in which the constituents of the animal's body are arranged; no "form" or psuchê need apply. A corollary of this, of course, is that the human soul cannot be related to the body in the way Aristotle supposed. Souls become either ethereal, non-physical substances, as Descartes believed, or nothing at all, as the materialist Thomas Hobbes believed. Since their day, philosophers have oscillated between dualism and materialism, a dialectic some think we can escape if we return to Aristotle. But it would be quite a trick to detach Aristotle's thoughts on the mind from the rest of his view, which, several centuries after Galileo et al., cannot help but seem quaint and indeed pre-scientific. The value of the De Anima might lie precisely in its providing a window on a worldview so different from our own.