Ethical concepts are, or purport to be, normative. They make claims on us: they command, oblige, recommend, or guide. Or at least when we invoke them, we make claims on one another; but where does their authority over us - or ours over one another - come from? Christine Korsgaard identifies four accounts of the source of normativity that have been advocated by modern moral philosophers: voluntarism, realism, reflective endorsement, and the appeal to autonomy. She traces their history, showing how each developed in response to the prior one and comparing their early versions with those on the contemporary philosophical scene. Kant's theory that normativity springs