This dissertation is intended as a contribution to philosophical debates about the relationship between law and morality. My basic claim is that this relationship is importantly illuminated in various ways and at different levels by an understanding of the way in which a theory of law necessarily incorporates a metaphysics of human nature. As we will see, sometimes this incorporation is explicit, as when Jeremy Bentham's psychological hedonism gives rise to an understanding of law's bindingness as grounded in peoples' causal history with incentives. More contentiously, I argue that though contemporary positivists take their account of law to be metaphysically noncommittal, views of what it is to be human agent continue to motivate, if implicitly, their positions. This is the case, I suggest, with H. L. A. Hart's analysis. I argue that Hart, in his various engagements with rival jurisprudential theories, draws tacitly on an underlying ontology of human agency. Likewise, these rival accounts often have reasons for rejecting Hart's analysis that go deeper than can be addressed at the level of substantive theory. In order to properly understand these debates, I suggest, we must understand better the relationship between a theory of law and the conception of human agency that underlies it.