In Part I, I shall offer an interpretation of Plato's theory of the tripartite soul as it is presented in the Republic. Two groups of claims are central to my interpretation. The first group concerns partition of the soul as such. Plato's theory, I shall argue, holds the embodied human soul to be a composite of a number of distinct and specifiable items. The theory takes it that impulses to act arise, not from the soul as a whole, but, in each case, specifically from some part of it. It is, moreover, part of the theory that while the embodied human soul can give rise to a desire for, and a simultaneous aversion to, one and the same thing, no individual part of the soul can by itself give rise to motivational conflict of this particular kind.
My second group of claims focuses on the lowest of the theory's three parts of the soul, appetite. I shall argue that it is part of Plato's theory that appetite is non-rational in the strong sense of lacking the capacity for reasoning. At the same time, the theory takes appetite, like the other parts of the soul, to be capable of giving rise to fully formed impulses to act, so that it can, all by itself, get a person to behave in some specific way or other. It can, for example, get Leontius to run towards a pile of corpses lying by the side of the road, so as to take a close look at them (Republic 4, 439 E 5–440 A 4). The notion of a part of the soul that is incapable of reasoning, but capable of giving rise to episodes of behaviour, even to episodes of human behaviour, sets the scene for the book's central theme: the idea, shared by Plato and Aristotle, that while reason can, all by itself, motivate a person to act, parts or aspects of the soul other than reason are equipped with non-rational cognitive resources that are sufficient for the generation of fully formed motivating conditions.
My main argument for the non-rationality of appetite, as Plato conceives of it, depends on my view of what Platonic soul-partition comes to. My argument, in a nutshell, is this. According to my view of partition, no individual soul-part can give rise to a desire for, and a simultaneous aversion to, one and the same thing. Plato conceives of appetite as being naturally attracted to pleasure (Republic 4, 439 D 6–8). If appetite is rational, it is capable of forming reasoned desires for what it takes to be better in the long run, and of forming reasoned aversions to what it takes to be worse in the long run. If so, it is vulnerable to just the kind of motivational conflict that Platonic soul-partition rules out at the level of individual soul-parts. For appetite's nature will saddle it with desires for pleasures that it may, if it is rational, at the same time be averse to, on the grounds that pursuing the pleasure in question would be worse in the long run. Therefore, Plato's theory of the tripartite soul is coherent only if he conceives of appetite as non-rational.
(p.10) Chapter 1 is introductory. It lays out in some detail what the rest of Part 1 is meant to establish, against the background of recent and not so recent literature on Plato's psychological theory. Chapter 2 offers an in-depth discussion of the Republic‘s argument for tripartition of the soul. The main purpose of that discussion is to argue for my view of what Platonic soul-partition comes to. Plato's argument for tripartition depends crucially on what is standardly referred to as the Principle of Opposites, which says that the same thing cannot at the same time do opposites in the same respect and in relation to the same thing. I shall argue that the context of the overall argument makes it clear that what this principle is supposed to mean is that the same thing cannot at the same time be the proper subject of opposite predicates that apply in the same respect and in relation to the same thing. I shall show, moreover, that Plato takes desire for, and aversion to, one and the same thing to exemplify a pair of opposite predicates that apply in the same respect and in relation to the same thing. He plainly accepts, furthermore, that it is a common occurrence for someone to desire, and at the same time to be averse to, one and the same thing. According to my interpretation, he is committed to the view that such motivational conflicts always reveal a partition of the soul, with one part being the proper subject of the desire, and another part being the proper subject of the aversion. He is also committed to the view that if a part of the soul is incomposite, it cannot itself harbour such motivational conflicts. And it can be shown that he conceives of the three parts of the soul that are argued for in Republic 4 as incomposite.
Chapter 3 defends my interpretation of Platonic soul-partition against the objection that a Platonic soul is not the right kind of thing for it to make sense to say of it that it genuinely has parts. It also addresses the philosophical cost of soul-partition, so understood. It does so by considering Socrates’ remark in Republic 10 that ‘it isn't easy for a composite of many parts to be everlasting if it isn't composed in the finest way, yet this is how the soul now appeared to us’ (Republic 10, 611 B 5–7). The chapter closes with a brief glance at Aristotle's psychological theory, by considering an Aristotelian concern about soul-partition. Aristotle thinks of the soul as, among other things, a principle that accounts for the unity of the organism it ensouls. However, for something to be a genuine principle of unity, it cannot itself be a composite. For composites stand in need of unification by something else. Aristotle's position on soul-partition will be a recurring theme of this book. We shall find that Aristotle is unwilling to commit himself to the view that the human soul is a thing of parts. One question this raises is whether Aristotle can consistently accept the Platonic analysis of human desire into three kinds without accepting the Platonic analysis of the human soul into three parts. I shall turn to this question in the book's conclusion; my answer will be affirmative.
Chapter 4 completes the argument for my view of what Platonic soul-partition comes to. It does so by disarming two prima facie reasons against it. One of these is that Plato, in Republic 8 (553 A 1–555 B 2), seems to describe a case of motivational conflict within appetite, and he seems to have in mind just the kind of conflict (p.11) that Platonic soul-partition, on my view, rules out at the level of individual soul-parts. There is good reason to think, however, that the motivational conflict that Plato is describing at 553 A 1–555 B 2 is supposed to be a conflict, not within appetite, but between reason and appetite, or between reason and spirit on the one hand and appetite on the other. Moreover, many scholars think that, in Republic 9, Plato implicitly attributes to appetite the capacity for instrumental reasoning. If they are right, this not only refutes my claim that Plato's theory holds appetite to be incapable of reasoning. It also throws in doubt my view of Platonic soul-partition. For if appetite is rational, it is vulnerable to motivational conflict of just the kind that, according to my view of partition, Plato's theory rules out at the level of individual soul-parts. So much the worse, one might think, for my view of partition. (Alternatively, so much the worse for Plato's theory.) I shall argue, however, that Plato neither says nor implies that appetite is capable of instrumental reasoning.
The chapter ends with some remarks about Plato's theory of human motivation, as it emerges from my interpretation of the argument for tripartition of the soul. One remark is forward-looking. This is that my interpretation presents Plato as operating with a conception of what is distinctive of rational motivation that is not only clear and robust, but also importantly continuous with Aristotle's conception of rational motivation. I shall turn to Aristotle's conception in Chapter 12. The main points of contact between Plato and Aristotle are, first, that rational motivation depends on thoughts to the effect that something or other is good, and, secondly, that it brings into play desires of a very special kind. These spring from, and are informed by, the subject's grasp of means–end, or ‘for the sake of’, relations. The formation of such desires involves the transmission of desire from A to B in such a way that B comes to be desired specifically as a means to, or for the sake of, A. (p.12)