Although Kant’s view according to which our theoretical representation of nature “coheres” with our representation of nature from the perspective of our practical needs culminates with the third Critique and the notion of reflective judgment’s principle of nature’s purposiveness, this chapter demonstrates that the origins of this view can already be discerned in Kant’s discussion of nature’s systematicity in the first Critique, namely, in his discussion of the rationalist notion of the Transcendental Ideal and his account of nature as a unified system of laws in the Appendix to the Dialectic. While for some commentators reason’s need for the unconditioned is exclusively a reflection of its practical need, I argue that that the notion of the metaphysical ground of the unity of nature is a necessary notion for reason in both its theoretical and practical functions and, moreover, that reason’s practical ends are presupposed in every theoretical investigation of nature.

In the first part of Chapter 8, we push forward to the very heart of speculative metaphysics, its account of God and the alleged proofs of God’s existence. We will reconstruct Kant’s derivation of the ‘transcendental ideal,’ that is, the idea of an ens realissimum, and argue that there is only one (abductive) argument for God’s existence that Kant regards as springing from ‘universal human reason.’ In the second part of the chapter, we return to Kant’s discussion of the metaphysical presuppositions of science in the Appendix to the Transcendental Dialectic, where Kant explains the tendency to make constitutive use of transcendental ideas and principles in scientific investigations.