A Look at St. Thomas' Fourth Proof for God - Summa Theologiae (PP, QII, AIII, I Answer That)
Act - Potency Gradations
Traditionally this Fourth way has been called the proof from "gradation." In reality, though, the first three proofs are also types of proofs from gradations in being. The difference is the aspect of being that is being analyzed as a gradation. It's important, though, to introduce what an "act-potency gradation" is. A gradation is, of course, a common quality which is expressed in different amounts, more or less. Act and potency are Aristotelian terms which describe the being of things. Actuality refers to the way that something actually exists right now, whereas potentiality refers to all the ways in which a thing could change in order to be, but currently isn't. Traditionally Aristotle identifies 10 categories of actuality, or 10 ways in which a things can be and be described. Act and potency, then, are ways of describing these categories in terms of more or less, thus implying a type of gradation in relation to the quality that's being considered. The First way expresses a gradation with regards the aspect of change that occurs in things. Some things have the potency to change a lot, while others less. Thomas' point is that for this gradation to exist of the potential for change in things, there must exist one thing which has no potential for change and is pure actual in its being. The Second way expresses a gradation with regards to causes. Small causes depend on greater causes, which depend on greater causes, which then depend on an ultimate cause which, by definition, depends on nothing else. The Third way expressed a gradation with regard to types of beings. Lower beings depend on higher beings for their existence. But there must be one necessary being which depends on nothing else but itself, by definition.
This act-potency gradation could look something like this if expressed in symbolic form.
The difference of the fourth way from the first three is that the quality being described by the act-potency gradation of being is not a material one, but a transcendental one. St. Thomas offers three examples of gradations - truth, goodness, and nobility (which some interpret to mean beauty). These are immaterial qualities in things that only the intellectual mind is able to comprehend, and traditionally known as the "transcendentals of being" because they are a part of every being. Again, they are not physical characteristics, but spiritual ones. Only the rational mind is able to perceive the intelligibility of truth, the intellectual desirability of goodness in things, and the harmony of beauty, in a universal way in things.
Thus, just like there is a gradation for material qualities in their changes, in the nature of efficient causes, and in the types of beings, there is a gradation in these transcendentals in things. Transcendentals are qualities of being, and thus are subject to the act-potency structure of things. All act potency structures conclude in the same way, which is that any composite of act-potency needs a reality of pure actuality to explain its existence. The transcendental compositions of act-potency in things is no different. For there to be a limited goodness in something, it depends on an ultimate goodness as its cause. This is because there is direct relation between the perfection of a transcendental quality and the perfection of the general being of thing. Thus, seeing lesser versions of these transcendental qualities shows a necessary dependence on an ultimate version, and thus on an ultimate being. This ultimate being is one of pure actuality, and that to which we traditionally give the name God.
Aquinas, Thomas. Summa Theologiae. Prima Pars, Q2, AIII, I Answer That.