Aquina’s cosmological argument states the existence of God, and claims that cosmos or universe exists, the existence of the cosmos has a cause, and that cause is God. Everything begin was cause by something else that happened before; therefore, either there is an infinite casual chain being extending backwards or there is a first cause, something that wasn’t caused by anything but caused something else. There is no infinite casual chain extending backwards, so there must be a first cause that was not caused by anything but started everything else. Therefore, God exists.
Cosmological argument attempts to prove God’s existence by observing the universe. Thomas Aquina addressed God’s existence to his systematic theology of the “quinquae viae”