The proof of God’s existence has been a matter of different kinds of sciences. Scientists, philosophers, theologians and others, all have attempted to find a certain basis of God’s existence, which would substitute the approach of accepting knowledge as is without doubting its origins. In a selection from Summa Theologica by Saint Thomas Aquinas, the author provides a list of five proofs of God’s existence. This paper summarizes the third proof for the existence of God, which uses the fact that “contingent” things exist as conclusive evidence for God’s existence.
As every proof Aquinas used was based on some known aspect, i.e. motion, efficient cause and etc, the third proof was based on the argument of the possibility and the necessity. In regards to possibility, Aquinas stated that there are things in nature that are either possible to be or possible not to be, because it is of the nature of things to exist and to be destroyed. Accordingly, as the things that are possible not to be at a certain point will change its state from possibility to actuality, i.e. in the long term they will vanish, things that of that nature cannot exist forever. From the aforementioned reasoning Aquinas concluded that in case everything had the possibility not to exist, at a certain point there would be nothing in existence.
Aquinas explains the absurdity of the previous statement in a way that, if at a certain point nothing existed, then nothing would start its existence, and thus nothing currently would exist. In such a way Aquinas led to a conclusion that there is a thing that is beyond the characteristic of merely possible to exist; a thing which existence is a necessity not a possibility. However, Aquinas stated that the necessity of such an object is either caused by another object or not. In the first case, if the necessity was caused by another object, then it would form an infinite chain. Based on the second proof, which is acknowledging that there is a chain of causes in the universe, where nothing is caused by itself, there certainly should be a first efficient cause that started this chain. Therefore, an infinite chain is impossible.
Thus, there must be a being which has its necessity not caused by another, but existing in its own nature and causing necessity to others. Acknowledging the fact that such a being exists leads us to the conclusion that this being must be God. God is the only thing that has the necessity in his own nature having the power to cause necessity in other things and start the chain.
It is obvious from Aquinas’ proof, that the main idea, either based on motion, the efficient cause, or the necessity, is that there is always a starting point of things, and no objects started existence from nothing. There were always the first mover, the first efficient cause, and the first necessary being which started the subsequent movements, causes and necessities. This necessity of the “first” in everything, led Aquinas to state that God exists.