treatise · medieval
Summa Theologiae I, Q.2, A.3 — Whether God exists
St. Thomas Aquinas · 1265–1274
In this article St. Thomas Aquinas asks whether God exists, and answers by offering five classical demonstrations drawn from motion, efficient causality, contingency, degrees of perfection, and the governance of the world. The ways do not replace faith; they show that belief in God is reasonable and that the mind can know that He is, even before knowing Who He is in Himself. Every Catholic studying natural theology returns here.
The question
Whether God exists.
Objection 1
It seems that God does not exist; because if one of two contraries be infinite, the other would be altogether destroyed. But the word "God" means that He is infinite goodness. If, therefore, God existed, there would be no evil discoverable; but there is evil in the world. Therefore God does not exist.
Objection 2
Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence.
On the contrary
It is said in the person of God: "I am Who am." (Exodus 3:14)
I answer that
The existence of God can be proved in five ways.
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another... Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
The second way is from the nature of the efficient cause... Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
The third way is taken from possibility and necessity... Therefore we cannot but postulate the existence of some being having of itself its own necessity... This all men speak of as God.
The fourth way is taken from the gradation to be found in things... Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.
The fifth way is taken from the governance of the world... Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
See also
Study this article alongside the life of St. Benedict, whose Rule orders a life under the God Aquinas demonstrates, and the longer tradition of the Summa as a whole.