My dissertation pursues the reverse order of explanation. I propose that we should ground the notion of being a reason in the notion of rational agency. To delineate such a view, we need a more substantive account of rational agency. I favor a constitutivist approach to rational agency according to which actions and beliefs are subject to certain norms (i.e. rational norms) simply in virtue of being the sort of things they are. I argue that the Aristotelian form of constitutivism I develop is preferable to Kantian constitutivism, as defended by C. Korsgaard and S. Engstrom.

On a Kantian picture, practical norms can be derived from the form of practical reason. According to Kantian constitutivism, this is because rational agency is constituted by practical norms, similar to the way that chess is constituted by its norms, i.e., its rules. Chapter I discusses Korsgaard's version according to which an action is the kind of thing it is because it performs its function, which is to make an agent a unified person. An agent constitutes herself through her actions. However, I argue that Korsgaard is committed to the implausible view that an agent loses its capacity to act when she does not exercise it properly. I also argue that her account of agency is not substantial enough to ground substantial norms. Moreover, associating one's capacity to act with sophisticated norms, Korsgaard is committed to a hyper-intellectualized account of action. This is a problem she shares with Engstrom.

Chapter II discusses Engstrom's view. According to Engstrom, practical norms are constitutive of one's capacity for practical knowledge (i.e. knowledge of what to do). That is, Engstrom thinks that to will is to exercise the capacity for practical knowledge (i.e. to will involves the judgment that the action is good) and one does not exercise one's capacity for practical knowledge properly (i.e. one's practical judgment does not amount to knowledge) if one's will does not conform to its form. Following Kant, Engstrom thinks that one has an implicit cognition of the form of one's will and is guided by it when one exercises one's capacity for practical knowledge. Discussing various forms of implicit knowledge, I argue that it is implausible to think that the form of the will is present in one's psychology even implicitly. Consequently, one cannot be guided by the form of the will.

The second part of this study concerns my positive account of the nature of practical reasons. Chapter III, which set the stage for my positive account in chapter IV, concerns the notions of function and the good. I argue that the notion of function cannot be reduced to biological or causal notions. The function of an item is usually its characteristic activity. I argue that the notion of function has a close connection to the notion of essence or life-form. The notion of life-form can be explained by facts about an organism's nature and environment which can ground an objective conception of that organism's good.

In chapter IV, I argue that actions aim at the good. Many philosophers, among them G.E.M. Anscombe, J. Raz, B. Willams and D. Davidson, who defend the doctrine of the guise of the good understand it as the idea that the object of intention is taken by the agent to be good. I argue that this is implausible. Further, I argue that the doctrine of the guise of the good should be understood in the same manner that we understand the thesis that beliefs aim at the truth. That is, we should understand it as saying that the formal object of a practical attitude is to bring about the good. This requires a teleological conception of practical attitudes. I argue for this teleological conception of practical attitudes on the grounds that it can explain why practical attitudes need rationalization. If so, a practical state is defective if it does not bring about the good i.e. if it does not bring about its object or if its object is not good. (Abstract shortened by UMI.).