The cosmological argument, as presented by Thomas Aquinas, is an argument for the existence of God that relies on the concept of causality. According to Aquinas, everything that exists must have a cause, and this chain of causality cannot go back indefinitely. There must be a first cause, which is what we call God.
Aquinas presents this argument in his work "Summa Theologica," in which he seeks to prove the existence of God through reason alone, without relying on revelation or faith. He begins by stating that everything that exists must have a cause, or else it could not have come into being. This is because, as he puts it, "nothing can be reduced from potency to actuality, except by something in actuality." In other words, for something to come into existence, there must be something that already exists that brings it about.
This chain of causality cannot go back indefinitely, Aquinas argues, because that would mean that there would be an infinite number of causes, which is impossible. There must be a first cause, which is what we call God. Aquinas explains that this first cause must be something that exists necessarily, rather than contingently, because it is the source of all other things that exist contingently. In other words, God must exist by necessity, rather than by chance or because of something else.
Aquinas also argues that this first cause must be something that is not itself caused by anything else, because if it were, then it would not be the first cause. It must be uncaused and self-existent. Furthermore, this first cause must be something that is pure actuality, or pure being, because it is the source of all other things that are not pure actuality.
Aquinas concludes that the first cause, or God, must be an unmoved mover, because it is the cause of all other things, but itself has no cause. It is also an unchanging being, because it is the source of all change, but itself is not subject to change.
In summary, the cosmological argument, as presented by Aquinas, is an argument for the existence of God based on the concept of causality. It states that everything that exists must have a cause, and this chain of causality cannot go back indefinitely. There must be a first cause, which is what we call God, and this first cause must be something that exists necessarily, is uncaused and self-existent, and is pure actuality or pure being.
