I begin with Kant's notion of a maxim and consider the role which this notion plays in Kant's formulations of the fundamental categorical imperative. This raises the question of what a maxim is, and why there is not the same requirement for resolutions of other kinds to be universalizable. Drawing on Bernard Williams' notion of a thick ethical concept, I proffer an answer to this question which is intended neither in a spirit of simple exegesis nor as a straightforward exercise (...) in moral philosophy but as something that is poised somewhere between the two. My aim is to provide a kind of rational reconstruction of Kant. In the final section of the essay, I argue that this reconstruction, while it manages to salvage something distinctively Kantian, also does justice to the relativism involved in what J. L. Mackie calls 'people's adherence to and participation in different ways of life'. (shrink)