In the ‘Appendix to the Transcendental Dialectic’ of the Critique of Pure Reason, Kant contends that the idea of God has a positive regulative role in the systematization of empirical knowledge. But why is this regulative role assigned to this specific idea? Kant’s account is rather opaque and this question has also not received much attention in the literature. In this paper I argue that an adequate understanding of the regulative role of the idea of God depends on the specific (...) metaphysical content Kant attributes to it in the Critique and other writings. I show that neither a heuristic principle of conceptual systematicity, nor conceiving God as a hypothesis of an intelligent designer, can satisfy the demands of reason to make the unity and necessity of the laws of nature intelligible. Regarding the positive account about the metaphysical content of the idea of God, I support my argument by referring to Kant’s pre-critical discussion of the usefulness of the conception of God for the project of science, and by expounding Kant’s critical account of the necessity of the laws of nature. Thus my account sheds light on the continuity of Kant’s conception of God and his appropriation of his own rationalistic metaphysics. (shrink)