Three Rational Proofs for the Existence of God

St. Anselm: An Ontological Argument (@1050)

St. Anselm, the Catholic archbishop of Canterbury and a Doctor of the Church, first formulated the Ontological Argument. This philosophical argument is perhaps the strangest and most hotly debated of the proofs. The argument has attracted the attentions of such notable philosophers as Immanuel Kant (who attacked St. Anselm’s proof) and G.W.F Hegel (who defended Anselm’s proof).

The proof is most notable because it alone claims to prove the existence of God by relying independently on human reason without the need for perception or evidence. The proof itself relies on the defined concept of God as a perfect being. St. Anselm’s proof is summarized here: 

1. God exists in our understanding. This means that the concept of God resides as an idea in our minds.

2. God is a possible being, and might exist in reality. He is possible because the concept of God does not bear internal contradictions.

3. If something exists exclusively in our understanding and might have existed in reality then it might have been greater. This simply means that something that exists in reality is perfect (or great). Something that is only a concept in our minds could be greater by actually existing.

4. Suppose (theoretically) that God only exists in our understanding and not in reality.

5. If this were true, then it would be possible for God to be greater then he is (follows from premise #3).

6. This would mean that God is a being in which a greater is possible. 

7. This is absurd because God, a being in which none greater is possible, is a being in which a greater is possible. Herein lies the contradiction.

8. Thus it follows that it is false for God to only exist in our understanding.

Hence God exists in reality as well as our understanding.


Study the above proof carefully. It is an intriguing proof because it states that God, a perfect being, must exist in all possible circumstances in order to satisfy the definition of his perfection. A God that can exist in only some circumstances, but fails to exist in others is a less than perfect being.


St. Thomas: The Teleological Argument (@1250)

The teleological argument, or argument from design, is also summarized by St. Thomas Aquinas in the Summa Theologica. Here is the extract from the Summa:

"The fifth way is taken from the governance of the world. We see that things that lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."

Perhaps this is the most common form of reasoning behind the existence of God. The average theist will argue for the existence of God with the teleological argument.


St. Thomas: The First Cause Argument (@1250)

St. Thomas Aquinas (1225-1274), whose writings on theological matters are regarded as authoritative by the Catholic Church, is the author of this famous argument (although he borrowed heavily from Aristotle's "unmoved mover" argument). The basic argument is very simple: everything in the world around us has a cause; but the chain of causation cannot extend back into infinity; therefore, there must be a first cause, and this is God. We may call this the popular version of the argument. Aquinas's full argument is somewhat more complex, however. Here is the original version from Aquinas's "Five Ways", as translated by Ronald Rubin: 


"In the world that we sense, we find that efficient causes come in series. We do not, and cannot, find that something is its own efficient cause — for, if something were its own efficient cause, it would be prior to itself, which is impossible. But the series of efficient causes cannot possibly go back to infinity. In all such series of causes, a first thing causes one or more intermediaries, and the intermediaries cause the last thing; when a cause is taken out of this series, so is its effect. Therefore, if there were no first efficient cause, there would be no last or intermediary efficient causes. If the series of efficient causes went back to infinity, however, there would be no first efficient cause and, hence, no last or intermediary causes. But there obviously are such causes. We must therefore posit a first efficient cause, which everyone understands to be God. 

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