In the 13th century, Thomas Aquinas gave three forms of the cosmological argument in his Summa Theologica as a part of his “five ways” of proving the existence of God.
Aquinas’s thinking was this: “Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must be traced back to God, as to its first cause. So also, whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change and fail. For all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle.”
Aquinas first tried to demonstrate God’s existence from motion. Motion is an effect and as such, needs a cause. According to Aquinas, “whatever is moved must be moved by another.” This chain of one thing moving another that moves another cannot regress infinitely.
As we have shown above, this is impossible. Thus, there must be a first cause that sets all others into motion — an unmoved mover. Although everything may be fully capable of functioning, without a first, uncaused cause to initiate the action, everything would remain idle and useless. Without the unmoved mover to open the lid, the universe would become like a wound-up music box that remained forever closed, motionless, and silent. Furthermore, to suggest the music box needs no unmoved mover to open the lid is to suggest that the wood and metal assembled themselves into the music box without the need of a craftsman. This first cause or unmoved mover is what we call God.
Aquinas employed a variation of this argument to arrive at his second proof. This argument, rather than being based on motion, is based on existence, or what he calls “efficient cause.” Everything that comes into existence owes its existence to something else. There is nothing that brings itself into existence or causes itself. Thus, existence is an effect of a cause that is itself an effect of a cause, and so on. But once again, we cannot trace this lineage of causes back infinitely. There must be a first cause to explain why any cause exists. This first cause must be a self-existent being that does not rely on anything for existence. This self-existent, non-contingent being is called God.
Third, Aquinas based an argument on the possibility of existence. Nothing we see in the universe has to exist. Everything we see could just as well not have existed. This makes everything that exists simply possible, not necessary. But something does exist. “Therefore,” says Aquinas, “not all beings are merely possible, but there must exist something the existence of which is necessary.” Thus we know that a necessary being must exist in order to account for the possible beings that do exist; it makes the possible beings possible. A being that is necessary for the existence of all things is called God.
This post is an excerpt from the Holman QuicSource Guide to Christian Apologetics by Doug Powell. It is used with permission. You can purchase this resource in its entirety here.