Something must cause this change, for nothing changes itself, so there is an external force causing the changes in the world. The external power behind change would not change itself. Aquinas considered this external force to be caused by god. Also, there are many things in motion and they must j=have been caused to move by something. Aquinas believed that the chain of movement cannot go back to infinity but instead he supports the idea of the Aristotle's unmoved mover- the cause of the original movement without being moved it. Aquinas believes this prime mover to be God.
"Now whatever is moved is moved by another, for nothing ca is moved except it is in potentiality to that towards which it is moved.".
Cause was the 2nd way according to Aquinas. He noted that all causation within the universe had to be caused by an original cause. Aquinas believed the first cause to be God. It was god who started and caused the beginning of existence. It was this first cause, which started the chain of causes that causes things to happen.
"Therefore it is necessary to admit a first efficient cause, to which everyone gives the name God.".
Contingency is the 3rd way of the 5. Based on the fact that everything in the world comes into existence and then dies, Aquinas concluded that there was a time when nothing existed. This lead to the argument that the force, which caused the universe to come into existence, must have always existed for nothing can evolve from nothing. The external force had to have been a necessary being which brought everything else into existence. This necessary being was, according to Aquinas, God. If God did not exist, then nothing would.
"Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist.".
Copleston is a 20th century philosopher who shared ideas with Aquinas. Coplestons argument is based on Aquinas" 3rd way - Contingency. They both come to a similar conclusion yet Coplestons argument is much more simpler.