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 Republic is an excellent resource for drawing a contrast between the rhetoric of the sophist and of the philosopher. Its first book includes a dramatic conflict between the sophist Thrasymachus and the philosopher Socrates, while its middle books provide a detailed account of the nature of each. The dialogue contrasts and compares them through both its drama and abstract descriptions. In this chapter, I argue that Plato develops his presentation of the philosopher and the sophist dialectically. The initial books deliberately set up Socrates to look a bit like a sophist and Thrasymachus to look a bit like a philosopher. While the middle books suggest that the philosopher and sophist stem from the same nature, Plato differentiates them by the philosopher's love of the forms and his possession of moral and intellectual virtues. However, because sophists do not even acknowledge that the forms exist, the philosopher is separable from the sophist only from the viewpoint of the philosopher. From the sophist's viewpoint, a philosopher is merely a deficient sophist. Philosophy is a normative rather than descriptive standpoint, grounded upon key moral and theoretical commitments.

This chapter proceeds in three main parts. First, I examine the way in which Book One presents Thrasymachus as similar in certain respects to philosophers and Socrates as similar in certain respects to sophists. Second, I trace the dialectical development of the philosopher and sophist from the Republic Books Two through Seven.

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:

Plato's Phaedrus contains Plato's most explicit and well-developed account of rhetoric. The dialogue provides an account of good rhetoric as philosophical psychagôgia, the leading of souls toward the forms. For the first time in the dialogues, we have a clearly articulated account of rhetoric in its positive sense. Socrates' account of good rhetoric describes good speech as an organic unity of collection and division. However, the dialogue's account of philosophical rhetoric in distinction from sophistical rhetoric is complicated when one considers the dialogue as a whole. Socrates' use of rhetoric in the Palinode shows that there is more to rhetoric than a technical method of organizing material according to kinds. In addition, Socrates' technical account of rhetoric is itself rhetorical in how it seeks to persuade Phaedrus to care about philosophy. Both Socrates' Palinode and his general description of rhetoric are rhetorical. Both are designed to lead Phaedrus' soul away from sophistical rhetoric and toward love of the forms. While my focus in this chapter is primarily on the contrast between Socratic and sophistical rhetoric, I also briefly address the issue of the dialogue form and its rhetoric of leading souls. The Phaedrus helps to tie up many of the concerns in the previous chapters insofar as the unifying feature of good, philosophical rhetoric is love: love of the forms and love of those to whom the rhetorician speaks.

The Apology is a dialogue that at first blush seems to separate Socrates from the sophists with ease. After all, he denies that he is a teacher, while the sophists claim to teach. Socrates says that he teaches neither natural science nor excellence (aretê); although he has followers, he does not take any payment from them (19d–e). Certainly, these elements separate Socrates from the sophists. However, the Apology's account of the differences between philosophy and sophistry – particularly between philosophical and sophistical rhetoric – is far more nuanced, for Socrates uses common forensic topoi in his speech, seemingly giving credence to the prosecution's insinuation that he is a “clever speaker” (deinou ontos legein) (17b). Moreover, as I argue herein, Plato's own rhetoric as the author of the dialogue follows closely in the footsteps of his sophistic predecessors. In particular, several elements of Plato's Apology closely mimic Gorgias' Defense of Palamades.

In this chapter, I examine how Plato as the author of the Apology uses elements reminiscent of the sophists in order to separate Socrates from them. While philosophical rhetoric bears many superficial similarities to sophistic rhetoric, it has a distinctive set of aims. My discussion of the differences between Socratic and forensic rhetoric falls into three main areas. First, Socrates' questions are aimed at making those whom he questions more virtuous.

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).

The first chapter laid out the perplexities or aporiai confronting the effort to provide accounts of soul and its attributes, perplexities resulting from endoxa that draw thought in opposed directions. Much of this reflection was methodological. A connected task for the beginning of an Aristotelian science is to review the substantive positions of predecessors. Here the endoxa are not as in chapter 1 general ones about what an account of soul and its attributes should be like, but the concrete accounts the predecessors give of the soul and its attributes. This further eliciting of endoxa continues the project of dialectic. The predecessors in their general notions and concrete accounts of soul raise perplexities and may offer ways to resolve them. In 403b20–22 Aristotle says that in order to be well provided (εὐπορεῖν) we are to go through the perplexities simultaneously (ἅμα διαποροῦντας) with the opinions (δόξας) of predecessors, that is, their views and difficulties to which they give rise. We may embrace (λάβωμεν) what the predecessors have said well but be on our guard against (εὐλαβηθῶμεν) whatever they have not said well (b23–24).

By speaking of his predecessors' doctrines along with their reasons for proposing them, Aristotle delves deeply into principles. As usual in his treatments of predecessors, he carefully orchestrates his approach so that he has profound preparation for elaborating his own positions that alleviate the perplexities.

Having determined in the previous chapter that there are external particular sensible objects, and having arrived at a general account of the perceptive power as the capacity to become likened to these sensible objects as they are sensible in actuality, Aristotle turns to what in general these sensible objects are. He indicates that he first (πρῶτον) considers the objects of each sense (καθ᾽ ἑκάστην αἴσθησιν) since if he is going on to treat each of the senses it is appropriate to understand faculties and operations in terms of their objects (418a7–8; cf. 415a16–22). “Of each sense” here really means each kind of sense, whatever kinds there are, since the Greek καθ᾽ ἕκαστον can refer to kinds as well as to particulars (see 414b32–33 for καθ᾽ ἕκαστον referring to kinds and 417b22 for τῶν καθ᾽ ἕκαστον applying to particulars). Talk of kinds of senses is pertinent insofar as different types of animals share kinds of senses and inasmuch as some sensible objects are perceived in virtue of themselves (καθ᾽ αὑτά), that is, in virtue of the kind of sensible object and the kind of sense. Aristotle must delineate the possible sorts of objects of the various kinds of sense perception and indicate which play the central role in the accounts of the senses. Only afterward will he be ready to treat more particularly each kind of sense, its special object and operation.

The chapter's concision is remarkable in view of its importance.