Cosmological arguments become very forceful when seen as arising from intractable features of unavoidable explanations.
1.) We can’t avoid giving broadly causal explanations for things that come to be. By “broadly causal” I mean to include accounts that are determined, probabilistic, freely willed, or any combination of the three. In the face of the question whether there is a cause in the broad sense of something that comes to be, the answer is (necessarily) “yes”. Otherwise there is no reason whatsoever – neither necessary, statistical, or willed – why something non-actual becomes actual, which is exactly the position one is in if he says there can be becoming without a pure actuality. The denial the principle extends far wider than just the theistic proof – in fact, it leads us to a sort of philosophical comedy – a detective who could stare at the knife stuck in a corpse’ s back and wonder if there was no cause whatsoever for it: not a murderer, not an accident, not anything. So what if the victim was walking around healthy just yesterday? The knife is just a brute fact, and that’s all there is to it.
2.) We can’t avoid taking what is per se and primo as the explanation for what is not such. Leaving aside the explanation of particular events as particular, explanation involves finding a predicate that is said of something both necessarily, convertibly, and at exactly the right level of universality (what Aristotle calls kath’ auto and katholou in c. 4-6 of the Book I A.Po.). The explanation of heat has to be something that necessarily hot (coffee or ice won’t work) convertible with heat, and also at the proper level of universality (to use Aristotle’s example from geometry, you don’t explain what the “figure with angles equal to two right angles” is if you say it is “isosceles” – since this is too particular – or “Euclidean”, since this is too general. You need to say “triangle”). But in light of this it becomes impossible to explain any transcendental quality without an appeal to something anyone can recognize as God. What else would necessarily exist, be the unlimited source of any finite good or truth, have an infinite dignity with respect to anything that has dignity, etc.? But this would all be tied up in being per se and primo existent, good, true, dignified, or any other transcendental quality of this sort.
This reduction of all things to the per se and primo is exactly the premise St. Thomas is assuming in the Fourth Way. One can deny this premise too, though to deny it universally would lead one to saying that, for all we know, the scientific explanation of heat might be coffee, the Persian Empire, or Mickey Mouse.