This thesis aims to provide a theory of human rationality which is consistent with, and supportive of, the naturalistic approach to philosophy of mind. I analyse why the common tendency not to furnish a systematic and rigorous account of rationality is philosophically inadequate - namely, because this overlooks the critical role which the concept of rationality plays in the 'principle of charity' (Quine 1960; Davidson 1984) in philosophy of mind, which is a central part of a unified naturalistic approach to philosophy of mind and rationality. I aim to contribute to the field by rectifying this omission. I identify the theoretical commitments of any naturalistic approach to the theory of rationality - in particular, I argue for the neglected fact that naturalism commits us to an instrumental theory of rationality. I defend the theory of instrumental rationality against its critics, by means of elaborating the implications of the notion - also frequently overlooked - that the concept of rationality is one which applies paradigmatically to agents, rather than to reasoning processes considered in isolation (for example). The resultant instrumental theory of personal rationality which I develop and defend is characterised by two main original (related) features: (1) Certain fundamental features of human rationality are identified as constitutive in nature and status - given that humans are agents, then it follows necessarily that our theory of rationality has these certain characteristics. (2) Full instrumental rationality demands rationality of the self, which introduces further rational constraints of consistency (for example). I build upon these two central features of my theory of rationality to demonstrate how this account can thereby rebut accusations of extreme relativism and inadequacy of scope. Thus I aim to show how my instrumental theory of personal rationality can provide a philosophically adequate account of the normative status of rationality within the remit of the naturalist approach, and thereby also furnish adequate philosophical support for a defensible naturalistic theory of the mind.