The chapter develops the question (raised in Chapter 4) about the precise way in which soul is supposed to play the role of the primary explanans of perception. It does so by bringing out the key difficulty that Aristotle faces and by analysing the three possible answers to this difficulty. The problem is that Aristotle seems to commit himself to three jointly inconsistent tenets: (i) the perceptive soul is the primary cause of perception; (ii) perception is passive; and (iii) the perceptive soul is impassive. These claims are inconsistent if it is true that (iv) there is no way for the soul of being the primary cause of φ-ing other than being the proper subject of whatever φ-ing consists in. Two dominant ways of resolving this problem, since antiquity, consist in denying Aristotle’s commitment to either (ii) or (iii). I argue that difficulties, both exegetical and philosophical, faced by each of these strategies are insurmountable. The third possible strategy starts from denying (iv). I trace such a strategy to the medieval idea of a sensus agens and argue that although the existing medieval (and later) versions cannot stand as such, the third strategy is nevertheless the most promising one.

The chapter provides a novel account of perceptual discrimination (krinein) in Aristotle. Against the widespread view that the most basic perceptual acts consist in noticing differences between two or more perceived qualities, I argue that discrimination is for Aristotle more like sifting, winnowing on a sieve: it consists in identifying – with an ultimate authority – the quality of an external object as distinct from any other quality of the given range that the object could have. The chapter further explores how the notion of discrimination is embedded by Aristotle within his causal assimilation model of perception. I argue that the central notion of a discriminative mean (mesotēs), introduced in An. 2.11, is intended to capture the role of the perceptive soul as the controlling factor of a homeostatic mechanism underlying perception. As such the notion lays the groundwork for resolving the apparent conflict between the passivity of perception and the impassivity of the soul (as analysed in Chapter 5). The prospect is further explored in Chapter 7. The present chapter concludes by arguing that Aristotle conceives perceptual discrimination as a holistic assessment of the external object acting on the perceiver, including those of its features which are not causally efficacious.

The chapter spells out the homeostatic model of how the soul is involved in perception introduced in Chapter 6, while addressing two main challenges for it. First, I argue that while the physiological details are not easy to tease out, there is no principal reason against Aristotle’s extension of the model from touch to other sense modalities. More importantly, I argue that we can understand the role of the perceptive soul as an extension of the model developed for the nutritive soul in An. 2.4 and based on Aristotle’s art analogy (from Phys. 2 and elsewhere). The upshot is that the basic perceptual acts are underlaid by bodily processes non-cognitively controlled by the soul. But while homeostasis is the aim of nutrition, in perception it only becomes a means for achieving something else, namely discrimination. The chapter closes by showing how the interpretation developed in this book pays off when it comes to understanding Aristotle’s two notoriously difficult concluding accounts: the account of perception as a reception of forms without the matter in An. 2.12 and the summarizing account of the cognitive soul in An. 3.8.

The chapter sheds fresh light on Aristotle’s account of perception by providing a novel analysis of the puzzles that he articulates within his discussion of the predecessors’ views, especially in An. 1. I argue that Aristotle takes the key insight of the traditional view that like is perceived by like to be expressed in the idea that the perceiver is like the perceptual object by which she is being affected. This idea seems inconsistent with the widely shared assumption that only unlike things can act upon each other. Aristotle’s predecessors were unable to resolve this tension (the notion of a generic likeness is of no help), but he believes that precisely this tension must be resolved by any successful account of what perception is. The only predecessor who at least hinted towards a resolution is Anaxagoras with his account of impassive nous (understood by Aristotle as a general account of cognition). But Anaxagoras failed to account for the causal aspects of cognition as a way of being affected by its object. Aristotle’s own account can be seen as an attempt to incorporate the true insights of both the view that in perception like is affected by like and the view that what perceives must be impassive.

The Introduction articulates the central question about the nature of perception and sets it within the explanatory project of Aristotle’s De Anima. What makes Aristotle’s account attractive, I argue, is that it strives to accommodate causal, qualitative, and relational features of perception. A central insight of Aristotle’s account is captured under the notion of perception as a complete passive activity, but that notion has, since late antiquity, appeared paradoxical to readers of De Anima and was, thus, systematically disregarded. The Introduction analyses the historical and philosophical reasons for this disregard. It further articulates the key dilemma pertaining to Aristotle’s view of the role played in perception by the soul: it should be the primary cause of an essentially passive and receptive activity, but it should itself remain unmoved and impassive; how can that be? Although this question has received relatively little attention among recent scholars, it is argued to be more crucial than the much-discussed issue of what happens in the perceiver’s sense organs. The final section of the Introduction outlines the argument of the entire book.