Thomas Aquinas adapted Aristotle and Avicenna's thinking to form his cosmological argument in which he states that the universe is the result of a 'first cause' that is itself uncaused, and this ultimate cause according to him, is God. The premise of his argument basically states that every thing that was once non existent has a cause. Since, according to the second premise -something that is finite and dependent (contingent) cannot create itself. In his third premise Aquinas stated that a causal chain cannot stretch back into eternity. This is why Aquinas argues --there must be a first cause-(God)-or there must be something that is not an effect. Other cosmological veterans speculating about God's existence - (like Aquinas) take the 'first cause' to be 'God'. Aquinas's argument is based on the fact that God has to exist due to the fact that the universe needs a cause to explain its existence. This cause is furnished by the concept of the creation of the universe by a supernatural being outside it, and this being is assumed to be God.
Aquinas's cosmological argument is based on Aristotle's belief in a 'first cause It was Aquinas who interpreted Aristotle's uncaused cause as 'God' by modifying his deistic view into a theistic one. According to his cosmological argument, every event has a cause; but every cause has been caused by another. To avoid endless regression, we have to postulate an uncaused and eternal first cause -(or necessary being that created itself)-and that is God. Aquinas wrote,
"We see in the world around us that there is an order of efficient causes. Nor is it ever
found (in fact it is impossible) that something is its own efficient cause. If it were, it
would be prior to itself, which is impossible. Nevertheless, the order of efficient
causes cannot proceed to infinity, for in any such order the first is cause of the middle
(whether one or many) and the middle of the last. Without the cause, the effect does
not follow. Thus, if the first cause did not exist, neither would the middle and last
causes in the sequence. If, however, there were an infinite regression of efficient
causes, there would be no first efficient cause and therefore no middle causes or final
effects, which is obviously not the case. Thus it is necessary to posit some first
efficient cause, which everyone calls 'God.'"
Thomas Aquinas, Summa Theologiae, c.1260 CE
In his book, 'The Governance of the World' Aquinas propounds five ways- of proving the existence of God. In the process Aquinas argues for the presence of a benevolent order in the universe that makes even unintelligent entities work towards some goal. On the basis of these phenomenon, Aquinas argues in favour of the existence that is an essential part of this benevolent order. Aquinas stated that -'Now whatever lacks knowledge cannot move towards an end, unless it is directed by some being endowed with knowledge and intelligence and this being we call God.'