The cosmological argument or first cause argument is that everything has a cause, causal chains cannot be infinite, and so there must be an uncaused cause at its origin. It is a posteriori argument (knowledge gained after experience) which attempts to prove that there is a rational basis for the belief in God.
Christian philosopher St Thomas Aquinas (1225-1274) came up with the cosmological argument not to prove the existence of God but rather solidify his already established faith through reason through looking at the cause of everything in the universe which he claims to be God. He does this through his ‘five ways’ however I’ll only be looking at the first three.
The first way is an argument explaining the ‘Unmoved Mover’. Here Aquinas explains that everything in the world which is in motion is subject to change and develops and it has to be put into motion and changed by something else. He believes that this chain of cause and effect are not infinite. By this logic everything that is in a state of movement must have been put into this state by a different object. For example: all trees began to exist at some point and the cause of the tree was the seed. This seed must have come from another tree however this chain cannot go on forever, someone or something that is not subject to change must have planted a seed. Because of Aquinas’ rejection of infinite regression this means that there must be a ‘First mover’ who is ‘unmoved’.
The second way makes a similar point however it replaces the idea of ‘motion’ with the idea of ‘cause’. Aquinas starts off by stating that nothing can be an efficient cause of itself; everything is caused by something else. Once again Aquinas rejects the Idea of infinite regression. Nothing can be the cause of itself- Nothing can create itself. There cannot be an endless string of objects causing other objects to exist, therefore, there must be an uncaused first cause