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. Yet, this 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, meaning from the world around us, and he limited himself, by and large, to arguments based on reason rather than revelation.
The Five Ways, as they are 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 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.
Thomistic philosophy is basically Aristotelian in methodology and point of view. It is empirical and realist. Thomas preferred an order of study that presupposed the liberal arts and mathematics, and he began with Aristotelian logic, principally On Interpretation and the Posterior Analytics; moved through natural philosophy involving all the natural sciences, including psychology; treated moral philosophy, including political science; and concluded with metaphysics, or first philosophy, which today would include epistemology and natural theology. Moral philosophy for Aristotle and Thomas Aquinas presupposes psychology and deals with human happiness, the goal of every person in this life, and the optimum (morally good) means of attaining that goal for the individual the family, and the body pol...