Here is an outlined version of Aquinas’s Argument from Degrees of Perfection for the existence of God, also known as the Fourth Way. This is just an outline. The full series of articles can be found here, here, here and here. Refer to those for the whole, in depth explanations and defenses of the various premises.
The argument itself:
- In each class of existing things, there is a possible hierarchy of degrees of perfection in terms of transcendental qualities such as being, goodness, and perfection.
- The entire set of all classes of all existing things likewise creates a hierarchy of all being in terms of those same transcendental qualities.
- Every being that has these transcendental qualities (being, goodness, perfection) has them as either intrinsic to/identical with their essence, or else derives them from some external source.
- The fact that there is such a hierarchy of being, with each level of qualitative existence containing a higher or lower gradation of degrees of perfection, makes it impossible that these beings could have the transcendental qualities as intrinsic to or identical with their essence.
- Thus, each finite being must have their transcendental qualities by participation, meaning that they derive them from some external source.
- Either the finite being will derive its transcendental qualities directly from a being which has these qualities essentially, or else it will derive them from a being which likewise has them only be participation
- To derive transcendental qualities from an external source constitutes an essentially ordered causal series.
- Hence, if a finite being derives its transcendental qualities externally from another being which has the qualities only by participation, there will be an essentially ordered causal series.
- Essentially ordered causal series cannot have an infinite or circular regress, and must terminate in some first cause.
- Therefore, there must be some first cause of all finite beings (beings which have their transcendental qualities only by participation), and this first cause must have the transcendental qualities essentially.
- A being which has the transcendental causes essentially will be identical to the transcendental qualities, i.e. will be Being Itself, Goodness Itself, Perfection Itself, etc.
- This Being will also be One, and will be Pure Act, thus making it immaterial, timeless, eternal, immutable, and personal (having intellect and will).
- This Being we can rightly call God.