De anima i may be offputting because it jumps quickly into consideration of the difficulties of the investigation and because it reflects upon Aristotle's predecessors of long ago. It may help to view this first book, and even the whole De anima, as a variation upon Socratic dialogue. Aristotle's treatment of soul is calculated and contrived as the written Socratic dialogues, as befits a systematic investigation. Such dialogues as Plato's Charmides, Laches, Euthyphro, and Meno start with scene setting leading to the classic “What is X?” question. Aristotle's first chapter sets the scene by preliminary inquiry into the location of the investigation and its aim (402a1–7); these opening remarks lead to the central question of the work: what is the soul (a7–10)? In the Socratic dialogue the interlocutor's opening answer to the “What is X?” question may be of the wrong sort, and Socrates must argumentatively expound upon how to go about defining X. Comparably Aristotle's first chapter considers issues surrounding definition of the soul. In Socratic dialogue the interlocutor offers a series of accounts of X, accounts delineating all the really likely answers, that Socrates defeats through elenctic argument; Aristotle in the rest of book 1 restates the philosophical tradition's prominent answers to what the soul is and its basic features, and their reasons for supposing so. He takes these promising lines of previous thought, these plausible endoxa regarding soul, as interlocutors and refutes them.

If one were to seek a definitive answer to the contrast between the sophist and the philosopher, one might naturally turn to Plato's Sophist. The dialogue enumerates a number of collections and divisions, ending with a specific definition of the sophist. The dialogue's final definition of the sophist has the Stranger declaring that they have reached the truth about his nature:

However, despite the Stranger's own sense of clarity about his definition of the sophist, Plato's own voice in the dialogue is more difficult to discern. Unlike most Platonic dialogues, in which Socrates is a major speaker, here Socrates sits silently at the feet of the Eleatic Stranger as he performs diaresis and explores metaphysics. The identity of this stranger is mysterious. While other dialogues such as the Theaetetus often give us detailed accounts of their character's personalities or biographies, the Stranger is never full explained. We know little about him, other than that he is from Elea.

Since chapter 3 has distinguished sense and thought, it has already begun the treatment of mind or intellect. Clearly chapters 4–8 complete the treatment. This section on the mind is much more tightly organized than it may appear. Chapter 4 sets out what mind is through consideration of the analogy and disanalogy with sense. The main assumption of this chapter is that mind can think all intelligible things, while each sense has a limited range. On the basis of this assumption mind turns out unmixed and separate. Nevertheless, humans acquire knowledge so that they somehow have the intelligible objects right in the soul, though hardly so as to restrict thinking but to foster it. This raises the question taken up especially in chapter 5 of what can get humans thinking if they already have intelligible objects somehow in soul. It will be seen that the very intelligible objects that we have in mind enable us to think. But they do not lead us always to be thinking since the phantasma resulting from sense is also needed to stimulate thinking. Chapter 6 will focus upon the various intelligible objects. Chapter 7 will defend the role of phantasia in initiating thinking. And chapter 8 will return to justify the initial assumption of the whole investigation, that is, that mind can think all things. Thus the treatment of mind is quite clear in its design.

The previous chapter argued that sense and thought differ.

Aristotle has justified the succession of the crucial powers of soul, nutrition and sense perception, and in the case of the latter both contact and distance senses. He has justified them functionally – that is, they are necessary and along with suitable bodily apparatus sufficient for accomplishing vital functions or ends of the living organism – and in this way he has demonstrated the accidents of the soul as following necessarily from the definition of the soul and its faculties (see 402a7–10). The operations of the living beings that are the accidents of being ensouled are relatively obvious, even if they require demarcation, so that it merely has to be established that the capacities of soul accomplish them, in conjunction with the body, and are needed to accomplish them. Thus, these faculties conduce to the plant or animal's nature being for the sake of something. The senses are necessary and sufficient for bestowing animal life upon a suitable body, and the contact senses primarily so.

Previously Aristotle suggested that plants among living things have a body too simple to support the sense of touch (434a27–28), and in setting out to prove that touch is necessary for animal life he contended that touch supposes a body that is not simple (434b8–11). He has yet, however, to argue against overly simple bodies for animals. Now he provides the argument for his earlier claims that too simple bodies will not uphold a sense of touch necessary for animal life.

Aristotle might have spoken of the faculty of soul that moves an animal in respect to its location after he dealt with sense perception, and in ii 3 it was seen that progressive motion extends to more animals than does intellection (consider the order of powers of soul in 414a31–32). But presumably since humans also have thought entering into their desire and locomotion, he has treated all the cognitive capacities before taking up those that cause voluntary motion. It would be clumsy to deal early with the voluntary motion of animals only to have to return to human voluntary motion later after the treatment of mind. And to treat phantasia first would also seem important, if this is central to the life of many animals (415a11) and even human practical thinking depends upon it (431a14–17, b6–10). Phantasia, it may be recalled, only became a topic in iii 3 in the midst of distinguishing thought from sense. Hence, the position in the text of this inquiry into capacity for progressive motion seems appropriate. Treatment of the cognitive capacities of ensouled beings is thus surrounded by inquiry into capacities for motion, that is, the nutritive capacity and the faculty for progressive motion. Moreover, this locomotive faculty may straddle faculties already considered more than is the case with the others considered. This allows Aristotle to raise some issues that lead well into the concluding chapters of the book.

The desiderative power has been established as that which moves animals. The cognition pertaining to progressive animal motion needs further attention since some animals are extremely simple while others are quite complex. Aristotle has just emphasized that there can be no desiderative power without phantasia and referred to calculative and sensitive phantasia (433b28–30). What if there are animals so simple that they completely lack phantasia, as was suggested in iii 3.428a9–11? What would be the implications for desire and animal motion? And what is calculative phantasia? Does the distinction of sensitive and calculative phantasia call into question the way phantasia was defined in iii 3? Aristotle must confirm that calculative phantasia does not interfere with that earlier account. Moreover, he should show that calculative phantasia fits with the view that desiderative power moves the animal: such phantasia is not moving the animal without involvement of desire.

What is it that moves the incomplete animals (τῶν ἀτελῶν), where by “incomplete” is meant lacking the full range of senses (433b31–434a2)? The animals under consideration have merely the contact senses, touch and taste (ἁφή in 434a1 may apply to both), but none of the distance senses that complete the possible sensitive component of animals. Such animals without distance senses, it will be seen in 434b24–27, do not have progressive motion. Might these animals also lack phantasia, and if so can they have appetite?

In ii 2 Aristotle has explored the cause of mortal life through investigating the various powers of soul and how they might or might not be separate from each other. His main concern was that the other powers not be separate from the nutritive life power. Now he considers more fully how the powers of soul are distributed to living things and why they unite in the soul. Previously he was arguing against multiple souls in the ensouled being, while now he argues that each kind of living being has a unified soul. If the powers of soul are in succession, then nutritive capacity ushers in life with possibilities for further sorts of life. The powers (δυνάμεις) of soul referred to are nutritive power (θρεπτικόν), perceptive (αἰσθητικόν), desiderative (ὀρεκτικόν), motive according to place (κινητικὸν κατὰ τόπον), intellective (διανοητικόν, 414a29–32). Different classes of mortal living things possess all, some, or one of these (cf. 413b32–33). Because he is considering how widely these faculties extend and which can be separate, he here lists them, unusually, in the order of decreasing extensiveness. Humans have all of these functions, some animals only some, and plants just the nutritive capacity. As more capabilities are added, the number of living things possessing them decreases. Aristotle reconsiders how these functional capacities separate and combine in the classes of mortal living beings to guarantee that he has dealt well with all the powers of mortal living things and that his definition of soul covers all the soul's powers.

In the previous chapter Aristotle offered arguments against all the obvious candidates to serve as the locomotive power. He ended up contending that mind is not the locomotive power because thought without desire does not cause motion and desire dominates intellect in incontinent persons. Yet neither is desire always determinative of motion because mind overcomes it in the continent person (433a1–8). But he begins this chapter suggesting that it appears that both desire and mind move animals according to place. He states, “It is apparent to be sure that these two are movers, either desire or mind, if someone would place phantasia as some sort of intellection” (φαίνεται δέ γε δύο ταῦτα κινοῦντα, ἢ ὄρεξις ἢ νοῦς, εἴ τις τὴν φαντασίαν τιθείη ὡς νόησίν τινα, 433a9–10). Both desire and mind can be in play, since he says “two” (and see a13 immediately afterward), though he may use the disjunction inclusively to allow for either …or, or both together, since one or the other might cause motion or both together, and even if these always somehow work together, they often seem to be in opposition. That phantasia can be called some sort of noesis was already suggested in iii 3.427b16–17 and b27–29. If nous includes phantasia, and beasts are moved by phantasiai, then nous may have a clear role in animal motion.

The De anima and Self-Knowledge

That which Aristotle investigates systematically in the De anima he calls by the name ψυχή (psyche), that is, soul. This term appears among a rich vocabulary available to the earlier tradition for related notions. In the earliest Greek literature that we possess, Homer's epic poems, the psyche is a kind of breath-soul escaping at death, the eschatological soul flying off to Hades' realm and retaining there merely a pale, shadowy existence. This may help explain why ψυχή could also be used for butterfly or moth (see Aristotle HA 551a13–14). Heraclitus and Plato prominently accept psyche to stand for the whole soul (see, e.g., DK 22B36, B45, B77, B107, B115, B117, B118, and Plato Apology 29d–30b, Republic 353d, Cratylus 400a, Laws 959a). They perhaps delight in suggesting ironically that the afterlife need hardly be shadowy as Homer depicts it and that many persons now upon this earth lead merely shadowy lives.

But why suppose that there is soul at all? The ancients observe or postulate certain operations and functions; for example, animals engage in voluntary motions and have perceptive capacities, and humans seem perhaps to have some capacity for survival after death. Soul is then posited as necessary for explaining such functions as their source or cause. Only that account of soul suffices that manages to handle compellingly the function or functions that soul is introduced to explain.

Though the divisions of Aristotle's treatises into chapters cannot be attributed to Aristotle himself, and even division into books may derive from editors, the present division of books 2 and 3 of the De anima, in spite of the fact that the subject is still sense perception, seems appropriate. Book 2 carries out completely the project of a general account of sense perception applying to the five particular senses (see 416b32–33 and 424a17–19). Chapters 5 and 6 suggest that this general account must concentrate upon sense in terms of the proper sensibles (see 418a24–25), and subsequent chapters elaborate upon perception of the proper sensibles through media and sense organs. Thus book 2 demarcates the formal and material components of sense perception of each of the five senses. Book 3 introduces a new line of reflection upon sense perception, turning to the interconnection of the five senses and their relation with other faculties of soul. The opening two chapters deepen the general understanding of sense perception as well as prepare for the coming discussion of intellect and progressive motion. Book 2 could seem quite narrow in its limitation of perception to distinct ranges of sensible objects such as white or sweet; the reflection upon the interrelation of the senses in book 3 enters somewhat more into the lived experience of percipient beings that includes perception of common sensibles, accidental sensibles, and the whole field of perceptible things.

If Aristotle has worked out his most common account of soul, and in doing so indicated that soul is the principle for all its capacities, he should now go on to the accounts of the soul's parts or capacities. He will begin with the most necessary of the soul's faculties, that which every mortal living being must possess, the nutritive capacity. The initial remarks about understanding of faculty by its operation and the operation in terms of its object explain some of his method but also allude to the topic of life since the various sorts of living seem to be operations of certain faculties. The nutritive capacity has an object upon which it works, food. In this way it prefigures the way the other faculties will relate to their objects. Moreover, the nutritive faculty breaks into subfaculties in connection with nourishment from food, growth, and reproduction. Here again there is parallel with the higher faculties, as sense breaks into five senses and mind divides into theoretical and practical mind. Aristotle also pulls together the subfaculties of nutritive soul by relating them all to saving the life of the living being. Most characteristic of life in all its forms may be this self-preserving in contrast with the typical self-destruction of motion. If life is activity in contrast with motion, this is as we should expect since activity has its end in itself and is always complete and continuable rather than on the way toward a limit and termination.

The treatment of touch completes the accounts of the five senses. The sense of touch presents a peculiar problem inasmuch as there is not a simple set of proper sensible objects, as there is merely color for sight or flavor for taste. Taste has a single contrariety, sweet-bitter, and sight the single contrariety white-black, which covers the entire range of types of sensibles. The tangible objects include several such contrarieties: hot-cold, wet-dry, hard-soft, and so on. Consequently, it is unclear whether there can be a single sense and sense organ for touch, as well as whether touch involves a medium at all, since it is a contact sense, and whether there can be but one medium for touch. Finally, it seems that the sensory apparatus in touch undergoes standard rather than nonstandard alterations. The flesh may surely be heated, cooled, dried, or moistened by the sensible objects that we feel, and perhaps when we touch something hard the compression hardens the flesh while touching the soft relaxes and softens the flesh. Touch may, because of such standard alterations, run afoul of the general account of perception in 418a3–6. If Aristotle can show that touch requires a single medium and sense organ, and that nonstandard alteration occurs regarding these, then he confirms the line he has taken throughout his earlier treatments. Since touch is the most necessary and fundamental sense, proof that it conforms to what is anticipated from the earlier accounts solidifies the entire Aristotelian framework.

We expect from book 1 that Aristotle will begin with the general account or definition of soul (see 402a7–10). Since soul is the subject matter of the present investigation, the definition of soul will provide the primary principle. Unlike most of his predecessors who concentrated exclusively on animal or even merely human soul, Aristotle aims for an account that applies as widely as possible, that covers every instance of ensouled being. He thus makes a decision regarding questions raised back in 402b3–10 about how wide the definition should be and what should be the order of the inquiry. His is not merely a general account of soul but the most general or common account. An advantage of such general definition is to keep the project at its proper level. Neither will Aristotle go below the level of the soul in an attempt to reduce soul to nonsoul; nor will he go above soul generally to speak only of mind. Since the definition is so general, applying to the various sorts of soul even beyond a genus such as animal, it will not be definition in the strictest sense (Metaphysics vii; 4.1030a11–12 claims that strictly only the species of a genus has an essence and definition). Nevertheless, the worth of this definition, besides the clarity it gives to soul as principle of body and the living being, is the platform it provides for accounts of the soul's faculties occupying the rest of the treatise.

Aristotle's accounts of the five senses can concentrate upon their proper sensible objects and the way they operate through a medium and sense organ. Since he has already developed a general account of a sense faculty in ii 5.418a3–4, “The sense power is in potentiality the sort of thing the sensible object is already in actuality,” all that is needed to develop a definition of a particular sense is insertion of the sensible object. For example, vision is in potentiality the sort of thing the visible object, that is, color, is in actuality. Similar accounts readily apply to the other senses (see esp. 422b15–16). Hence these definitions do not receive much explicit formulation. Instead Aristotle concentrates on what the sensible object is and how the sensory apparatus including the medium and sense organ permits the operation of sense perception such that the sensible object can be perceived as it is. Thus he justifies his general understanding of sense. Since his general account of sense is open to many objections from the phenomena of the particular senses, Aristotle, wishing to make his case as strong as possible, introduces these phenomena to show that they fit his account. Hence each of the proper senses is treated for its own sake and for sustaining the general account of sense, to which ii 12 returns after the chapters on the five senses.

Though at the end of chapter 1 Aristotle announces satisfaction with the outline sketched of soul (413a9–10), this definition stands in need of shoring up at a crucial point, the assumption that only living natural beings are ensouled. He has stated, “By life I mean both nutrition through itself and growth and decline” (412a14–15); what justifies the assumption that soul links with life and with nutritive life in particular (a concern going back to 411a26–b5)? Is the account of soul adequately embracing of the various sorts of life so that it is truly the most common account of soul (κοινότατος λόγος, 412a5–6)? This chapter takes up the causes of life and argues that soul serves well as such cause. If having nutritive capacity is sufficient for life, and the other faculties of soul also sufficient for life are in succession such that they presuppose the nutritive capacity, then nutritive capacity that belongs to every ensouled being is the necessary and a sufficient condition for mortal life. And since the other sorts of life that he considers link with nutritive life, soul can account for all the kinds of life of mortal beings. Were plants denied life and animals the only ensouled beings, a quite different account of soul would perhaps be needed concentrating upon cognition, so that the hylomorphic theory would be less likely to emerge.

Having completed the accounts of vision, hearing, and smell, Aristotle turns from the distance to the contact senses. He has investigated sensible objects and the distance senses' need for media to show that the sense power is potentially like what the sensible object is in actuality. The medium undergoes the assimilating action of the sensible object, as consequently does the sense organ, rather than serving as mere thoroughfares for effluences flowing from objects. His predecessors turn all sense perception into some sort of touch; that is, corporeal effluences or projectiles having magnitude, figure, and motion – common sensibles – give rise to what appears to be perception of proper sensibles (De sensu 442a29–b12). The accounts offered of the distance senses have attacked this approach. Can he follow through with the contact senses and reconceive them so that touch will in fact be the fundamental sense without succumbing to the predecessors' position? The contact senses should also require media in potentiality to their sensible objects. But taste and touch make a medium seem unlikely if the tongue or flesh is a sense organ tasting or feeling through direct contact. Aristotle argues instead that the tongue or outer flesh is part of the medium for taste or touch, the true organ being farther within.

Beyond cohering with the accounts of the distance senses, treatment of the contact senses has a key role in preparing for the final definition of sense.

Near the De anima's beginning, after indicating how fine and important inquiry into the soul is, Aristotle comments about the extreme difficulty of the enterprise: “In every way and altogether it is most difficult to gain any conviction concerning it” (402a10–11). If Aristotle then manages to develop a compelling account, we must be impressed and pleased with the accomplishment. But examination of the text may prove unsettling because it seems frequently obscure, and continued study may disclose discouraging tensions and disagreements. What are we to make of this work?

Thinkers prior to Aristotle audaciously sought comprehensive understanding of the world and its prodigious array of phenomena, living beings holding special fascination for them. The tradition of the history of philosophy initiated by Aristotle suggests that philosophy very soon turned its attention from the near at hand to the wider cosmos, and only later with Socrates focused on the human life (see Metaphysics 982b11–17 and 987b1–4). In fact, however, the human being and life-bestowing soul were of interest right from the start. The first philosophical fragment, that of Anaximander (DK 12B1), compares the scheme of the universe to the system of human justice, and Anaximenes (DK 13B2) has the human soul unifying us much as the cosmos is unified. Surely Xenophanes and Heraclitus, as well as Parmenides, Empedocles, Anaxagoras, and Democritus, reflect deeply upon human life. With Socrates and Plato the reflection intensifies and perhaps turns in some new directions.