Abstract

After presenting the “first and more manifest way” of proving the existence of God by reason alone in Summa Theologiae Ia, 2, 3, Saint Thomas Aquinas continues this project by turning in the “Second Way” to what he somewhat enigmatically calls “the nature of the efficient cause.” The greatest obstacle to understanding his Second Way, though, is determining precisely what Aquinas means by “the nature of the efficient cause” and “an order of efficient causes,” and how the Second Way is distinct from the First and Third Ways. This paper shows that instead of purporting to demonstrate God's existence as the cause of metaphysical being, as is commonly supposed, Aquinas is actually searching for the ultimate cause of immanent activity in rational beings. It argues for this reading both from textual evidence internal to the Second Way and in the First Way.