The question of proving the existence of God by natural reason is at the core of what has come to be known in Thomistic circles as Natural Theology (as opposed to the supernatural Theology of Sacred Scripture). Unless God is known to exist, one cannot have any other knowledge about Him. However, Aquinas has more to say about proving the existence of God than just the Five Ways of the Summa Theologiae.
For Aquinas, the question of proving the existence of God is always bound up with the question of how, and to what extent, we can know God at all. Often before he undertakes to prove the existence of God by reason, he feels it necessary first to show that His existence can be known without reliance on faith and scripture, but also that His existence is not self-evident but does require argumentation. It is clear that when he argues that God’s existence is not self-evident, he is opposing what has come to be known as The Ontological Argument of St. Anselm.
Below are links to texts of Aquinas in which he considers how we can know that God exists. Some of the texts below (On Being and Essence, On the Power of God) were not put forth as “proofs,” but they have nevertheless been cited by Thomists as containing Thomistic Proofs for the Existence of God. (The numbers in parenthesis refers to the dates the works are believed to have been written.)
Commentary on the Sentences of Peter Lombard, Book I, Distinction 3, Question 1, (1254-6)
- Article 1: Whether God Can Be Known by the Created Intellect.
- Article 2: Whether the Existence of God (Deum Esse) Is Known Intuitively (Per Se Notum).
- Article 3: Whether God Can Be Known by Man through Creatures.
- Article 4: Whether Philosophers by Natural Knowledge Knew the Trinity from Creatures
On Being and Essence, c. 4 (1254-6) – (The Man/Phoenix Argument)
Disputed Questions on Truth, Question 10, Article 12 (1256-9)
- Whether the existence of God is known through itself (per se notum) by the human mind like the first principles of demonstration which one cannot think not to be.
Summa Contra Gentiles, Book I (1258-64)
- Chapter 10: Of the Opinion of those who say that the Existence of God cannot be proved, being a Self-evident Truth
- Chapter 11: Rejection of the aforesaid Opinion and Solution of the aforesaid Reasons
- Chapter 12: Of the Opinion of those who say that the Existence of God is a Tenet of Faith alone and cannot be demonstrated
- Chapter 13: Reasons in proof of the Existence of God
Disputed Questions on the Power of God, Question 3 (1265-7)
- Article 5: Whether There Can Be Anything Which Is Not Created by God
- Article 6: Whether There Is Only One Principle of Creation
Summa Theologiae, First Part, Question 2 (1266-68)
- Article 1: Whether God’s Existence is Self-Evident (Can one doubt it?)
- Article 2: Whether It Can Be Demonstrate That God Exists (Or is faith necessary?)
- Article 3: Whether God Exists (The Five Ways)
Web Resources on the Five Ways
Commentary on Aristotle’s Physics (1269-70)
Commentary on Aristotle’s Metaphysics (1269-72)
Compendium of Theology, Chapter 3 (1269-73)