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.