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 starts by outlining the version of direct realism endorsed by Aristotle. I argue that he was committed to uncompromised realism about perceptible qualities and to the view that we immediately perceive the bearers of these qualities without any need of further synthetic acts. These features highlight the difficulty of capturing the explanantia of perception. Two notions key to that endeavour are those of mediation and discrimination. The chapter provides a novel analysis of mediation (for discrimination, see Chapter 6), arguing that, for Aristotle, media are – more or less perfect – qualitative conductors. Furthermore, the chapter addresses the existing debate about what, according to Aristotle, happens in the sense organs when we perceive. I argue that the dilemma governing this debate between spiritualism and materialism (either ‘literalist’ or ‘analogical’) is a false one. Tertium datur, and this alternative turns out to be precisely the view Aristotle embraced: perception consists of a thoroughly material process, but what this process results in must not be a standing material likeness (which would mark the end of perception because like cannot be affected by like), but a dynamic ‘phenomenal’ likeness – the presence of a quality of the perceived object which remains to be precisely a quality of that object.

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 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.

In this text, of which we have translated a large excerpt, Olivi defends his own version of the identity theory of the soul and its powers. On this version, the soul is a bundle of powers (plus spiritual matter), and so is identical to the entire collection of its powers (plus spiritual matter). To defend this view, Olivi first considers and rejects another version of the identity theory, on which the soul is a single power capable of eliciting all of the different vital acts associated with a living being. His main argument against this position is that diverse forms of production or of activity require powers that are themselves diverse by nature or essence. Olivi then attacks the distinction theory developed, for instance, by Aquinas, arguing that, among other things, this theory conflicts with our conception of ourselves as essentially free and rational agents. Next, Olivi criticizes Bonaventure’s distinction theory, arguing that it is impossible for the soul’s powers to be substances and yet dependent on the soul itself. Finally, Olivi puts forward his own preferred bundle theory of the soul and its powers.

In this text, of which we have translated a large excerpt, Henry of Ghent rejects the view defended by Albert and Aquinas according to which powers are accidents distinct from the essence of the soul. Instead, he maintains, the soul is its powers “through its essence”. To show this, Henry devises an argument directed against Aquinas’ claim that a power must be in the same category as its act. The upshot of the argument is that this claim must be rejected because, along with some premises widely accepted by medieval Aristotelians, it leads to a (vicious) infinite regress. Additionally, Henry develops his own theory of the powers of the soul. While he thinks that the soul is its powers through its essence, he contends that the soul’s powers do add something to the soul, namely, relations to their acts. For example, the intellect is the soul as related to the act of thinking, whereas sight is the soul as related to the act of seeing. On the basis of this relational account Henry discusses in this text the distinction between those powers of the soul that are tied to a bodily organ, such as sight, and those that are not, such as the intellect.

In this text, Duns Scotus asks: does the image of the Trinity in the rational soul consist in three really distinct powers? His answer is in the negative. The powers of the soul, he maintains, are really the same as the soul, but formally distinct from it as well as from one another. To develop this view, Scotus first refutes several alternative theories, including Aquinas’ distinction theory and Henry’s relational account of powers. In his refutation of Aquinas, Scotus provides a discussion of the Category Argument, arguing that it confuses two distinct senses of the term ‘potency’, ‘potency’ understood as power and ‘potency’ understood as a non-actual mode of being. Against Henry, Scotus argues that the view that the powers of the soul are the soul as related to different acts entails that these powers must always be actualized. To develop his own account of the soul and its powers, based on the formal distinction, Scotus draws on the notion of unitive containment and his account of the transcendentals. He argues that the soul is explanatorily prior to its powers, arguing that it exists at a “first instant of nature” while its powers exist at a “second instant of nature”.

In this text, James deals with the question of whether the will moves itself. In order to examine this question, he thinks it necessary to first develop a theory of the soul’s powers in general. At the heart of this general theory is a (then) unorthodox view about the granularity of the powers of the soul. James thinks that there are, in addition to our generic powers to think and see, a myriad of fine-grained powers in the soul, such as the power to think about cathood or the power to see red. James does not venture to say just how fine-grained these powers are, claiming that only God knows this. Drawing on terminology due to Simplicius, he calls a generic or coarse-grained power of the soul a “general aptitude” (idoneitas generalis), while he calls a fine-grained power a “special aptitude” (idoneitas specialis). James also argues in this text that an aptitude, whether general or special, is an “incomplete act”. By characterizing a power as an incomplete act, James is claiming that it is an incomplete version of the operation that it can bring about. So, for example, Socrates’ aptitude to think about cathood is an imperfect version of his act of thinking about cathood.

In this text, Ockham deals with whether memory, intellect, and will are really distinct powers. He answers in the negative. After presenting first Aquinas’, then Henry of Ghent’s, and finally John Duns Scotus’ views in some detail, along with replies, Ockham presents his own uncompromising identity theory of the powers of the soul. Based on the principle of parsimony, he argues that the rational soul is identical to the intellect and the will. By transitivity, this entails that the intellect is identical to the will so that the rational soul is a single power to engage in acts of thinking and and willing. Fleshing out his view, Ockham also gives a sort of rule for knowing when distinct cognitive and appetitive powers must be posited, and when not. According to this rule, if everything outside of a cognizer or desirer remains the same, and the cognizer or desirer is able to have an act of one power, while being unable to have the act of another power, then those powers must be distinct. For example, some people may be unable to see while being able to hear and other people may be unable to hear while being able to see. It follows that sight and hearing must be distinct powers.

In this text, Durand rejects a common assumption undergirding much of the Latin medieval debate about the relation between the soul and its powers. According to this assumption, which we can call the uniformity assumption, all powers of the soul that bring forth vital operations, such as the intellect and sight, bear the same relation to the soul. Identity theorists maintained that all of these powers are identical to the soul, while distinction theorists held that all of these powers are distinct from the soul. Against this consensus, Durand argues that some powers of the soul are identical to the soul, while others are distinct from it. More precisely, he advocates an identity theory with respect to the vegetative powers but a distinction theory with regard to the sensory and rational powers. He motivates his identity view regarding the vegetative powers with the argument that these powers have substances as their effects and the premise that causes must be like their effects. To motivate his distinction theory regarding the sensory and rational powers, Durand does not draw on Aquinas’ Category Argument, which he thinks is unsound.

In this text, Aquinas discusses the question of whether the powers of the soul are the same as the soul. The text is part of Aquinas’ analysis of Augustine’s doctrine of the image of the Trinity. Like his teacher Albert the Great, he argues that the soul and its powers are distinct, and, like Albert, he holds that the soul’s powers are to be viewed as necessary accidents “flowing” from the soul. But Aquinas goes beyond his teacher in an important respect. He devises a new argument in favor of the distinction theory—an argument that we may call the Category Argument. The argument goes like this. A power must be in the same category as its act. All acts of the soul, like thinking, seeing, etc. are accidents. Hence, the powers of the soul must be accidents too. Aquinas justifies the major premise that a power must be in the same category as its act by appeal to what we might call a Causal Proportionality Principle. According to this principle, a cause must be like its immediate effect, where the cause is a power, and the immediate effect is its operation. The Category Argument proved very influential for the subsequent debate over the relation between the soul and its powers.

In Ordinary Question 4, of which we have translated a large excerpt, Thomas of Sutton argues that the soul and its powers are distinct. Sutton’s strategy for defending the distinction theory is, roughly, two-pronged. First, Sutton develops his own conception of power and act. Second, he aims to show that Aquinas’ arguments for the distinction theory, in particular the Category Argument, are sound, despite objections to the contrary. Key to Sutton’s own conception of power and act are two ideas. The first is that a power is a kind of possibility. Specifically, a power is a possibility that remains when it is actualized. The second idea is that powers and acts are mutually exclusive kinds of being. No act is a power, and no power is an act. Since the soul is a kind of act according to the Aristotelian view, this entails, for Sutton, that the soul is not its powers. In this text, Sutton also seeks to rebut Henry’s regress argument against Aquinas. He argues that this argument is predicated on a mistaken conception of how necessary accidents or propria relate to their bearers.

The introduction describes the scholastic Latin debate over the relation between the soul and its powers in the late thirteenth and early fourteenth century. It shows that this debate concerned the question of how a living being’s natural kind, which is determined by its soul, and its kind-specific powers are related. Is a living being’s soul the very same thing as its kind-specific powers, or is its soul perhaps something more basic upon which its kind-specific powers depend? The introduction shows that there were two overarching answers to this question in the thirteenth and fourteenth century: the identity theory, according to which the soul and its powers are identical, and the distinction theory, according to which the soul is distinct from its powers. The introduction first highlights that the debate between identity and distinction theorists already arose in the twelfth century in response to Augustine’s doctrine of the image of the Trinity. It then traces, in broad strokes, the development of the debate from the twelfth century until 1250. Finally, it discusses the phase between 1250 and 1320 focusing on the reception of Aristotle’s theory of the soul in the Latin West.

This text of Bonaventure’s gives a nice indication of how Aristotle begins to influence the debate about the relation between the soul and its powers in the Latin West around the 1250s. In this text, Bonaventure is still very much in dialogue with the earlier debate. He recounts the early twelfth-century identity theory and earlier versions of the distinction theory, and he argues in favor of the version of the distinction theory defended by early Franciscans like Alexander of Hales. As Bonaventure states this latter theory, the soul and its powers are one “in substance” but differ “in essence”, the essential difference having to do with the fact that the powers of the soul, unlike the soul itself, are relations. But Bonaventure also introduces a philosophical innovation into this theory—one that draws on Aristotle. He argues that the powers of the soul, despite being relations, are special types of substances, namely, substances “by a tracing back” (per reductionem). He understands by “substances by a tracing back” imperfect occupants of the category of substance.

In this text, Albert deals with Augustine’s theory of the image of the Trinity. An examination of this theory leads him to an investigation of the relation between the soul and its powers. Albert contends that the soul and its powers are distinct. He maintains that the soul’s powers are propria, that is, necessary accidents, and following Avicenna he claims that they “flow” from the essence of the soul. In this text, Albert also considers the identity theory, on which the soul and its powers are the same entity but rejects it because it “borders on heresy”. He argues that the identification of the soul and its powers is perilously close to the identification of essence and power in God. Finally, Albert invokes the Boethian notion of a “power-whole” (totum potentiale) to develop his own account of the soul and its powers and to make sense of the Augustinian claim that our rational soul is an image of the Trinity. Albert was one of the main defenders of the distinction theory in the second half of the thirteenth century, and his version of this theory influenced Aquinas.

In this text, Godfrey asks whether a created substance can be the immediate principle of its operation. ‘Power’ and ‘immediate principle of operation’ are synonymous terms here. Hence, the question that Godfrey raises is this: is a substance a power? Godfrey’s answer is nuanced. Like Albert and Aquinas, Godfrey adheres to the view that powers like sight and the intellect are propria or necessary accidents distinct from the soul. However, he openly rejects Aquinas’ Category Argument for the distinction theory because he finds Henry’s regress argument against the claim that a power must be in the same category as its act convincing. The regress argument also leads Godfrey to argue that there is, in addition to such powers as the intellect and sight, what we might call a higher-order power of the soul. This higher-order power is the soul’s power to bear such powers as the intellect and sight. Unlike the intellect and sight, this power to bear powers is identical to the soul, Godfrey thinks. Thus, Godfrey defends the Thomistic distinction theory; but he also makes a concession to Henry by arguing that the soul is at least one power through its essence, namely, a higher-order power.