The first two chapters have demonstrated that for Aristotle nomos in the focal sense is a standard for conduct derived from the practical reason of an architectonic legislator. Legislative expertise, on this view, is a privileged sub-branch of true political expertise and it enacts laws with a view to the human good inclusive of the promotion of the conditions for virtue and well-being. The full implications of Aristotle’s thesis that lawmaking is a part of political expertise can only be understood, however, by reference to the claim that legislators should and do enact laws (nomoi) relative to the constitution (Pol. III.11, 1282b8-13, IV.1, 1289a14-22). In its simplest sense, the constitutional relativity of law entails that legislators should and do enact laws that are consistent with the priorities of their regime. More fundamentally, the constitutional relativity of law reflects the dependence of law on the explicit or implicit conceptions of the good life which inform alternative arrangements of ruling offices. In this chapter, I argue that the constitutional relativity of law is best interpreted through the lens of Aristotle’s theory of explanatory causes. On this interpretation, the politeia is the formal cause of the polis and it is structured by the conception of eudaimonia prevalent among the dominant political element. Section 1 defends both the general claim that Aristotle’s constitutional analysis is informed by his theory of explanatory causes and the specific claim that the politeia is a formal cause. In section two, I then connect the status of the politeia as a formal cause to the different conceptions of eudaimonia which determine diverse regime-types and their laws. Finally, in section 3, I examine whether the constitutional relativity of law entails that all law is necessarily partisan by considering its applicability to the best regime.