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Reason, Desire, and the Will: In Defense of a Tripartite Moral Psychology


Which aspects of our psychology are most central to explaining our intentional actions, and how should we conceive of them in light of their abilities to play those explanatory roles? These are key questions in moral psychology, and my dissertation tries to answer them, or at least to provide a beginning. As with much else in philosophy, the basic contours of this debate first came to us in Plato. Though I am not primarily concerned with the historical details, the initial argument of the dissertation and its distinctive approach reflect these Platonic origins in an interesting way.

In Plato’s Protagoras, he presents Socrates as having an intellectualist moral psychology; that is, as claiming that all intentional action is motivated by a belief about what is best. This leads him to argue against the possibility of weakness of will. Plato himself later rejected this view, most notably in the Republic. There he argued that properly accounting for psychological conflict required dividing the soul into three “parts” - rational, spirited, and appetitive. I argue for what can be seen as a contemporary analogue of this later Platonic view; in particular, I argue that making sense of important aspects of our agency requires thinking of ourselves as having three distinct motivational capacities, which I identify as reason, desire, and the will.

I begin by arguing that we can only act akratically, that is, against our judgment about what we ought to do, if there is a way to act for a reason besides our faculty of reason. This is because in such cases we have already formed a judgment about what we have most reason to do, and there is nothing besides this that can plausibly be attributed to an act of reasoning. And yet we do act akratically, and when we do so we act on the “weaker reason”. So there must be some other way to act for a reason than through reason. An obvious suggestion is that we act in such cases through desire. However, once we introduce this division between reason and desire, I argue that we will need a third, which I call the will - a capacity to consciously decide to act a certain way when reason and desire are insufficient or in conflict.

Though the appeal to desire in the initial argument in some ways seems obvious, to speak of desire, especially as a motivational capacity alongside reason, is to be embroiled in controversies that have dominated philosophical thinking about action since the early modern period. So I dedicate three chapters to defending the approach to desire appealed to by my tripartite theory. In the first, I argue against the “Humean” dispositionalist account of desire in favor of an evaluative conception. In the second, I argue that most recent evaluative accounts either fail to show how desire is really evaluative or else are overly intellectualized. In the third, I argue that we should think of desire as a form of responsiveness to apparent reasons, and show how this fits with both animal as well as human desire.

The last part of the tripartite view to be discussed is the will, which I introduce by way of a discussion of intention. I start by arguing that intention is a type of commitment to action that cannot be reduced to beliefs or desires, though in many cases beliefs and desires lead directly to intentions. Once we have this distinction between beliefs and desires and intentions, we can say more explicitly what the will is - it is a capacity to decide, where that involves the deliberate act of forming an intention. The basic claim of the tripartite view, that there are three distinct sources of motivation, can now be put in terms of intention, i.e., as saying that there are three distinct routes to an intention - evaluative judgment (a conclusion of practical reasoning), desire, and decision.

In the final chapter, I briefly apply the view to the problem of identification, an issue in moral psychology which might initially seem to be particularly difficult to account for on the tripartite view. On the contrary, I argue that the tripartite view has certain advantages over other approaches, and sketching what I call the character view of identification allows me both to review the basic shape of the tripartite view and demonstrate once more its wide appeal.

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