Aquinas did not think the finite human mind could know what God is directly, therefore God's existence is not self-evident to us. In other words, he rejected Anselm's ontological argument. So instead we must infer God's existence indirectly, from his effects which are more known to us.

 The first four arguments are generally considered to be cosmological arguments. The fifth argument is considered to be a teleological argument. The proofs take the form of scholastic arguments.

The Argument of the Unmoved Mover

In the world we can see that at least some things are changing. Whatever is changing is being changed by something else. If that by which it is changing is itself changed, then it too is being changed by something else. But this chain cannot be infinitely long, so there must be something that causes change without itself changing. This everyone understands to be God.

Aquinas uses the term "motion" in his argument, but by this he understands any kind of change, and more specifically a transit from potentiality to actuality. Since a potential does not yet exist, it cannot cause itself to exist and can therefore only be brought into existence by something already existing. When Aquinas argues that a causal chain cannot be infinitely long, he does not have in mind a chain where each element is a prior event that causes the next event; in other words, he is not arguing for a first event in a sequence. Rather, his argument is that a chain of concurrent effects must be rooted ultimately in a cause capable of generating these effects, and hence for a cause that is first in the hierarchical sense, not the temporal sense. His thinking here relies on what would later be labelled "essentially ordered causal series" by John Duns Scotus. This is a causal series in which the immediately observable elements are not capable of generating the effect in question, and a