 The mere fact of supervenience could not play this sort of role: after all, the supervenience of the ethical is compatible with the idea that everyone ought always to do what I want them to do. However, Sidgwick points to an important idea: that we expect there to be a rational explanation for any ethical fact. One ambitious way of developing this idea has been suggested by Nick Zangwill (2006). According to Zangwill, a central conceptual constraint on ethical reasoning is the “because constraint”: when we judge something to be wrong (or to have another ethical property), we are committed to its having this property because it has some other property. Zangwill claims that this principle “either is, or explains” ethical supervenience (2006, 273). And Zangwill goes on to argue that this constraint has striking epistemological implications: he claims that it entails that our only epistemic access to facts about the distribution of ethical properties is by knowing about the distribution of base properties, and knowing ethical principles that link the presence of base properties to ethical properties. He then argues that our knowledge of these ethical principles could itself only be a priori (2006, 276). If Zangwill is right about this, then the a priori character of moral epistemology can be derived from claims about the supervenience of the ethical.