Having traced mind's essence, operation, and objects, Aristotle needs to give more attention to thinking's initiation. Possession of knowledge puts universals in the soul that provide for thinking at wish. But what can provoke such wish, and in general how does dispositional knowledge lead to actual thinking? This is the role of phantasmata. This has been hinted at previously (see 403a8–10, 427b16, and perhaps 430a24–25), but he now needs to offer fuller consideration. He should not merely resort to phantasmata as a desperate assumption but argue for their role in thinking. In this chapter Aristotle begins with an obvious case, practical thinking and its initiation by phantasia, and then goes on to other types of intellect. His guide is the way sense perception of the pleasant or painful causes desire and pursuit or avoidance. As sense has desire and motion concatenated with it, so practical thinking has phantasmata, desire, and action linking with it. He then expands upon this case by further comparison of mind with sense. If sense is a unity, because (1) the action on the medium affects the sense and causes desire and pursuit or avoidance and (2) the five senses lead to a central sense and thus are subfaculties of the sense faculty, mind also is a unity requiring phantasmata along with its thinking and the mind is composed of subfaculties for practical thinking, theoretical thinking, and mathematical thinking. What applies to practical thinking will apply analogously to these other sorts of thinking.

Having sketched the views of the predecessors and why they hold them, Aristotle criticizes them for the rest of book 1. This is Aristotle's version of Socratic cross-examination. Chapters 3 and 4 challenge the views of those explaining the soul as mover in motion; chapter 5 attacks those focusing upon the soul's role in cognition. Cognition enters chapter 3 to some extent as well, because cognition may be a sort of motion. The soul might be in motion either because as mover of the animal it has motion of its own that originates motion in the body or because cognition is some sort of motion. This prominence of motion helps explain why 405b31 announces that soul's involvement with motion should be considered first (cf. 403b27–29). All the criticism of predecessors will be dialectical. Aristotle cannot simply demonstrably refute the endoxa but only argue against them on the basis of other endoxa. Dialectic lacks firm starting points, instead using some endoxa to challenge others. Yet in spite of being dialectical, Aristotle's arguments end up with the definitive conclusion that the predecessors' positions are impossible (see, e.g., 405b31–406a3). How can such a solid conclusion emerge from dialectical consideration?

Aristotle, it will be seen, uses a barrage of arguments. It may well provoke suspicion that so many arguments enter the attack against his predecessors' views. Were there a decisive argument, we might expect it to suffice, but Aristotle frequently resorts to a series of arguments. Plato's Socrates occasionally does much the same himself.

Two main tasks remain to complete in the treatment of soul. Near the beginning of the De anima it was announced that the investigation of soul is to disclose its nature and essence along with its accidents (402a7–8). These accidents are especially the operations of living things: nourishing themselves, perceiving, walking, swimming, flying, thinking, and so on. Presumably these somehow have to be demonstrated as following from the accounts of the soul and its capacities (see 402b16–403a2). Moreover, in book 2 in 413b9–10, 413b33–414a1, and 414b33–415a1 Aristotle announced that additional discussion is forthcoming about why the soul has certain capacities in succession. Why does nutritive capacity have priority over all other capacities of soul, and similarly all the other capacities presuppose the capacity for sense perception, while touch is the most fundamental sense? Now the anticipated treatment appears in confirmation of his whole approach to the soul and the soul's relation to body. If certain faculties of soul are necessary in succession, then their operations, the main accidents of the soul, are demonstrated as necessary. Achievement of this demonstration supports the definitions upon which it is based.

The argumentation here is teleological and hypothetical; that is, certain parts must be present if certain functions are to be performed and natural ends to be attained. The ends at issue are life and the good life that entered into the very definition of soul in ii i.

Having completed his general account of the nature of mind and the way this makes thinking possible, Aristotle considers in some detail the basic objects of thought: those about which we think truly as the special sensibles are those perceived truly. His treatment of intellect further resembles the inquiry into sense perception, since, after treating the nature of sense in ii 5, he discusses the various sensible objects in some detail in ii 6, and now after dealing with mind in iii 4–5, he takes up the objects of thought in iii 6. As in ii 5 there is some preliminary consideration of the sensible objects (see 417b19–28), so in iii 4.429b10–22 there is brief attention to the objects of intellection, though the fuller treatment occurs here in iii 6. Also the way thought draws its objects together as one will receive attention in iii 6, somewhat comparable to sense's drawing things together as he discusses in iii 2. Aristotle has been speaking rather as if all knowledge were the theoretical knowledge of essences. Thought may be about these primarily, but it also extends to more objects. Presumably he supposes all thinking explicable, much as all sense-perceiving, if he can account for its most basic kinds and objects. In addition, by showing the possible breadth of thought, he begins to justify his initial assumption from iii 4 that thought can think all things.

The first part of chapter 5 completes the examination of the self-moving number theory of soul. In words reminiscent of 408b32–409a1, Aristotle announces:

We are getting the result of the previous examination of soul as a number that moves itself. “Just as we said,” appearing prior to any indication of what has been said, can refer to the places in which Aristotle has spoken of fine-grained body (λεπτομερής), that is, 405a5–7 and a21–22, or to the passage in which the units in the self-moving number were likened to small corpuscles (409a10–11), or to Democritus's way of having soul as mover of body (406b15–22 and 409a11–15), or to the passage opening the reflection on the self-moving number conception (408b32–409a1), or, perhaps most likely, to the whole previous discussion of self-moving number (408b32–409a30). Resembling many of the thinkers who sought for soul the subtlest and most fine-grained body, the self-moving number view has soul as number composed of units. Aristotle has compared these units to points and even tiny corpuscles, as if Democritus reduced his soul atoms to points (see 409a10–12).

Aristotle has pursued two lines in considering sense perception, that which goes through the five proper senses and their sensible objects, media, and organs, and that which, avoiding duplications by arguing for just five senses and showing that perception of perception and discrimination of objects of the several senses require no additional sense, eventuates in a unification of sense. This latter line of reflection takes sense perception to the limits of its extension so that the realm of sense can be clearly demarcated. There are but five senses, and perception of perception and discrimination of all the objects of sense take place within the realm of sense. Humans generally combine their sense with thought and speech, however, and this makes thought hard to distinguish from sense. Plato may have been the first carefully to distinguish sense (αἴσθησις) from reasoning (συλλογισμός) and opining (δοζάζειν), yet perhaps even Plato was more concerned to disentangle thought from sense than to determine just how far sense can go. It remains, then, for Aristotle to stake out the whole realm of sense, as he has in this work up through iii 2, to prepare to investigate thought. There was little question about the respective turfs of nutrition and sense perception, even if there may be doubt as to whether some simple organisms are plants or animals (see, e.g., HA 588b4–589a9 and PA 681a15–b13).

The opening question of this chapter, whether another sense is needed to explain the way we perceive that we perceive or the sense itself perceives its own perceiving, connects with the previous chapter's argument against more than the five senses. If self-awareness of the senses requires a further sense, there would be more than five. The previous chapter asked whether there is some proper sense for perceiving the common sensibles; Aristotle seems to be asking here whether there is some additional proper sense for perceiving proper perceiving. The previous chapter started to show unification of the senses in some common sense; this unification here receives important verification. If the senses are all united in and working through a central sense, we may say either that a sense perceives itself or that the central sense is aware of sensing, without supposing any senses beyond the five. Unification of the sense power also accounts for perception that goes beyond the proper sensibles to the perception of common and accidental sensibles, and the discrimination of the various proper sensible objects. The chapter's order of treatment is well conceived because the perception of perception with which it begins is presupposed by the discrimination of the sensible objects of different senses with which the chapter ends. The order supports viewing five senses as subfaculties of a common, united sense power, and that sense is thus unified is vital to any adequate understanding of sense.

Color vision contributes greatly to the case for a sense medium. Hearing tests the case. Its object, sound, seems to travel through the air, so it is unlikely to be an affective quality of some substratum, as color is such a quality of a surface. Thus sound may well be that horror for Aristotle, an effluence. And bodies striking each other set up motions in air or water, that is, in the medium, but sound is heard rather than these motions. In this way Aristotle's realism, that perception of proper sensibles perceives things just as they actually are, is seriously tried by hearing because sound rather than the motion is heard. He will meet these challenges head on.

Commentators observe that the treatment of hearing and sound is more thorough than some of the others, and this is due to sound's seemingly different status from the other proper sensibles. Only by clarifying the origin of sound can Aristotle reflect upon what sound is and its medium, thereby handling the mentioned difficulties. Not until the De sensu does Aristotle discuss the origin of the other sensibles – except for tangibles in On Generation and Corruption 329b5ff. and Meteorology 340b5ff. and 385a1ff. – but having treated sound here in ii 8, he need say little in De sensu about its origin.

The treatment of nutritive life ultimately disclosed the way all the subfaculties work at saving life. This will also turn out fundamental for the treatments of sense and thought. When these cognitive faculties actually cognize their objects, their doing so will not be any sort of change or destruction of the faculty but its saving as the kind of being that it is. Perceiving and thinking are kinds of life, and life is activity complete and continuable. But Aristotle must now earnestly confront the difficulty of fitting treatment of cognition within the framework of physics, aframework that speaks of movers and things moved that is rather inapt for soul that might be better served by talk of activity complete at every moment. He therefore clarifies actuality and potentiality so far as needed to speak of sense perception, and such that he avoids a large break from physics.

Making the treatment of this chapter hard is the difficult relation of soul and cognition to physics. Aristotle has compelling reasons for offering claims that he also has to call into question. Book 1 has challenged any view that has the soul inmotion. So sense perception and thinking dubiously are kinds of motion, and he will himself in ii 5 elicit reasons for denying that they are motions at all. Yet if he needs to have them as ways of being acted upon by their cognitional objects, then in line with his natural science they should be motions caused by a mover. This serves to explain their origination and basis in reality.

Having worked out the outline account of each of the particular senses (καθ᾽ ἑκάστην μὲν οὖν τῶν αἰσθήσεων εἴρηται τύπῳ, 424a15–16), Aristotle is ready to pull together a refined universal account applicable to every sense (καθόλου δὲ περὶ πάσης αἰσθήσεως, 424a17). Such a universal account was begun back in ii 5 where he said that sense power is in potentiality the sort that the sensible object is already in actuality (τὸ δ᾽ αἰσθητικόν δυνάμει ἐστὶν οἷον τὸ αἰσθητὸν ἤδη ἐντελεχείᾳ, 418a3–4). Now that his treatment of the particular senses has dealt with sensible objects, media, and sense organs, and he has developed the account of sense as a mean suited for discriminating sensible differences, he can provide a more illuminating definition of sense. He will give an account in terms of receptivity to sensible forms. The question pervading this chapter is, What is this receptivity such that sense perception results? Why are all bodies not similarly capable of being acted upon by sensible forms? Acknowledgment is made that the sense media along with the bodily sense organs receive some action of the sensible objects though differently from the sense.

Aristotle starts with the general account and then offers some explication:

The two senses already considered, sight and hearing, contribute most to the higher life of animals. Sight especially facilitates recognition of things by disclosing many differences (see Meta. 980a21–27), while hearing permits communication by voice for the greatest learning (980b22–25). The treatment of these distance senses shows the need for media and how sense perceives what is there in actuality to be perceived. The sense organ that is much like the medium receives the action of the sensible object by way of the medium, rather than being bombarded by effluences. Smell is also a distance sense, but more closely tied to the contact senses than the other distance senses. Odor and flavor are linked so that smell is somehow connected with taste, that is, a contact sense. Smell and taste may be particularly conjoined for animals that only smell by way of respiration through the throat. As a distance sense with close ties to the contact senses, smell is a sort of intermediate (μέση) of the senses (see De sensu 445a4–16). Typically lists of the five senses place smell between the distance and contact senses (e.g., 424b23). That there are two contact senses and three distance senses, with one of the distance senses standing between the two groups drawing them together, indicates some natural ordering of the five senses and possible completeness of the set of five.

The connection of smell with contact sense and its relative imprecision in humans put it last among the distance senses.

The previous chapter emphasized that mind that is part of the soul must be unmixed and separate to think all things. This, it was argued, is consistent with the possibility of thinking, for both mind and its principal object, where these are theoretical knowledge and its object, share the condition of being unenmattered. Aristotle now clarifies just what it is that acts in mind to originate thinking if human mind is primarily potentiality to think all things. All he has to explain is what enters into occasioning this thinking, not to go into detail about the way it arises or to account for the development of concepts or knowledge. He merely has to follow out the implications of his remarks ending the previous chapter. Since theoretical knowledge is the same as the knowable, whereas the external sensible object acts upon the sense to make it operate, the intelligible object may give rise to thinking by way of the knowledge already within the soul. He needs to show that the mind can get itself thinking without seeming an impossible causa sui or self-mover.

Aristotle had said in 430a5–6 that one might consider why we are not always thinking. Divine beings will ceaselessly be thinking, but this does not seem to apply for humans. The reason must be that mind that belongs to soul, as he has established, is potentiality for thinking, and whatever is a potentiality can be or not be (see Metaphysics 1071b13–14, b19).

In arguing that it is impossible for the soul to be in motion (see 406a2–3), Aristotle has criticized those predecessors conceiving the soul as a magnitude itself in motion thereby moving the body. Chapter 4 continues the criticisms with regard to a theory that might be supposed very close to Aristotle's own. The view of the soul as a harmonia (ἁρμονία), attunement, or concord of the body may advance beyond the crude mobile soul, for example, the Democritean spherical soul atoms or the Platonic world soul moving the heavens by way of its rotational motion. A harmonia is not itself in motion and will not travel from instrument to instrument, as soul transmigrating to any chance body (see 407b20–23). The harmonia view connects soul closely with body without making soul a magnitude like body. Soul rather serves as proper attunement, synthesis, or ratio of the various contraries that compose the body. Since right above in 407b23–24 Aristotle suggested that the soul might be the form of the body, and form could be some sort of synthesis or ratio (logos) of the body, harmonia could be the body's form and converge with Aristotle's own view. He must distinguish this view from his own.

The attack on the harmonia view constitutes a crucial part of the overall argument. Aristotle's previous rejection of the soul as a magnitude in motion, that is, a substantial bodily being in rectilinear or circular locomotion, leaves open the possibility that the soul might instead be an attribute or accident of the body, much as color or three cubits (cf. 406a16–22).