In this thesis, I apply teleofunctionalism to a current debate concerning the normativity of practical rationality. Assuming teleofunctionalism is the correct theory of mental phenomena, I argue that it can provide a promising account of the normativity of practical rationality. This claim is motivated by the idea that a capacity to represent internal states, external states, and relations between these states as reasons for action has a teleofunction, and is thus a source of normativity. This teleofunction is marked by a distinctive causal role that reason-representation plays in action. Although I argue that this capacity developed out of processes of biological natural selection, the content of representations of reasons for action produced by the mechanisms underlying this capacity need not be determined solely by biological selection. In an effort to naturalize normativity in this way, I discuss the relation between biological-functional normativity and the normativity of rationality itself.