St. Thomas Aquinas has written several important works. Some of them are: The Disputed Questions on the Power of God, Exposition of Dionysius on the Divine Names and Disputed Questions on Spiritual Creatures. Most of Aquinas's works have been written to try to prove the existence of God. Aquinas has been a firm believer that everything had to have a creator and the only possible solution would be something called God. It is with this idea that Aquinas's Third Way was written.
In his De aeternitate mundi contra murmurantes, Aquinas insists that human reason cannot prove the impossibility of an eternally created universe. Once again Aquinas has written with the certainty that God has to exist in order to have created the universe. There is no doubt in Aquinas's mind that everything was created for a reason and that reason was God's will.
In the text Medieval Philosophy, St. Thomas Aquinas' article entitled Does God Exist? , Aquinas tries to show the different steps that can prove God does exist and that the world had to have been created by God.
This article is comprised of five different ways in which Aquinas tries to prove that we can be sure that God exists. In the third way, Aquinas tries to show that God exists if it is true that we exist.
In the article Aquinas says that it is possible for things to exist and for them not to exist. He means that living things will one day become non-living things. Aquinas believes that all things can not be mortal because if it were true, then at some point nothing would exist. What he means is if all things were living things, meaning that they had to be mortal, then at some time these things must also become non-existing.