Thomas Aquinas wrote the Summa Theologica between 1265 and 1273. The third article of the Summa attempts to demonstrate in a rational manner the existence of God. The article consists of five short, almost cursory proofs, totalling less than three pages, and yet it remains one of the most influential pieces of scholastic theology ever written. Aquinas' general method was to try to show how God can be known from his effects, i.e. the world around us, and he limited himself, by and large, to arguments based on reason rather than revelation.
The Five Ways, as they have come to be called, are grounded in Aristotle's Metaphysics. Aquinas' mentor, Albert the Great, was one of the first scholars to put pagan philosophy to work in the service of Catholic theology and Aquinas carried this approach on to its zenith. It has been argued that the Five Ways are structured in such a way as to reflect Aristotle's Doctrine of the Four Causes: the First and Second Ways deal with two different aspects of efficient causality, the Third Way deals with material causality, the Fourth Way deals with formal causality, and the Fifth way deals with final causality.
Probably the most famous (and obvious) example of Aquinas' reliance on Aristotle can be seen in the first Way, in which Aquinas uses Aristotle's notion of the unmoved mover to prove the existence of God. In Metaphysics Beta, Aristotle argued that change implies the existence of an ultimate and unchanging source of change, since an infinite regress of causation is impossible. He says, "If there is nothing eternal, then there can be no becoming; for there must be something which undergoes the process of becoming, that is, that from which things come to be; and the last member of this series must be engenerated, for the series must start with something, since nothing can come from nothing." When we examine Aquinas' First Way, the argument from motion, the similarities to the latter view b...