Reasons and reasoning were central to the work of Paul Grice, one of the most influential and admired philosophers of the late twentieth century. In the John Locke Lectures that Grice delivered in Oxford at the end of the 1970s, he set out his fundamental thoughts about these topics; Aspects of Reason is the long-awaited publication of those lectures. The focal point is an investigation of practical necessity (the necessity of 'I must not torture' or 'I must go to law school' for example). Grice contends that practical necessities are established by derivation; they are necessary because they are derivable. Aspects of Reason sets this claim in the context of an account of reasons and reasoning. This allows Grice to defend his treatment of necessity against obvious objections, also revealing how the construction of explicit derivations can play a central role in explaining as well as justifying thought and action. Grice was still working on Aspects of Reason during the last years of his life; unpolished as it is, the book provides an intimate glimpse into the workings of his mind. This rich and subtle work, powerfully evocative of its author, will refresh and illuminate many areas of contemporary philosophy.