Thomas Aquinas on the Existence of God: A Demonstration

The Need to Demonstrate the Existence of God

Thomas Aquinas posited the need for a demonstration of the existence of God. According to some opinions, such as those of Saint Anselm, such a demonstration is unnecessary since the existence of God is clear and evident, and only what is not clear and evident needs proof. However, Thomas Aquinas's opinion differs. According to him, a demonstration of the existence of God is necessary precisely because it is not self-evident that God exists.

To explain this statement, Aquinas distinguishes between two types of self-evident propositions:

• a) Propositions self-evident to us (humans): Where we know the subject and predicate and immediately conclude that the predicate is included in the subject. For example, "men are rational animals" or "triangles have three sides."
• b) Propositions self-evident in themselves but not obvious to us: These are propositions where the predicate is included in the subject, but we do not see this inclusion clearly and evidently.

The proposition "God exists" is self-evident in itself because existence is a property included in the essence of God. However, it is not clear to us because our mind is unable to see the subject "God" clearly, nor the nexus or relationship between the subject, God, and the predicate, His existence. Thomas Aquinas states that if our understanding were like that of angels, we would see as clearly that God exists as we see that triangles have three sides. However, human understanding is limited and, therefore, unable to see this clearly.

Therefore, since the existence of God is not evident to humans, a demonstration is needed. Now, is this demonstration possible? It is possible. According to Thomas Aquinas, a demonstration is a rational operation that obtains a conclusion from premises used as a basis and foundation, showing the cause from the effect (principle of causality).

Two Types of Proofs for God's Existence

Traditionally, attempts to prove God's existence have used two types of proofs:

• A priori (or propter quid): These start from the idea of God and descend to His effects. Saint Thomas rejects this type of argument, specifically the one proposed by Saint Anselm in the 11th century, known as the "Ontological Argument." This argument runs as follows:

1. 1st Premise: All men have the idea of God as the most perfect being that can be thought.
2. 2nd Premise: The most perfect being must exist both in thought and in reality because otherwise, there would be a being more perfect (one that also existed in reality) than the most perfect, which is contradictory.
3. Conclusion: God exists.

• A posteriori (or quia): These start from created things and go back to God as the ultimate cause of all that exists. If God created the world, He must have left some mark or particular label on it, just as the work of an artist somehow expresses the peculiarities of its author.

For Thomas Aquinas, only the latter, the a posteriori argument, is adequate to prove that God exists.