The Introduction states the book’s main purposes in examining Thomas Aquinas’s (1225–1274) famous five arguments for God’s existence, or “Five Ways,” in Summa theologiae Ia q.2 a.3. Even philosophically trained readers find them difficult to understand and often misread them. There are few extensive books in English solely on the Five Ways in the last few decades. Reading the Ways requires familiarity with positions Aquinas holds in his metaphysics, theory of causation, logic, semantics, theory of knowledge, philosophy of nature, and other areas of philosophy. This book recovers that necessary background for the reader. Careful attention is given the arguments’ premises and the reasoning behind them. Ample consideration is given to historical and contemporary objections to the various Ways and how Aquinas might respond. The Introduction closes with a summary of the book’s nine chapters. It is hoped that readers will think along with Aquinas on issues surrounding God’s existence and reach greater clarity in their own conclusions.

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.