The standard Catholic (Roman or Orthodox) understanding of the Divine Nature is that it is a simplicity. The standard complaint …
“And this all men call God.” Such were the words of St. Thomas Aquinas at the end of each of his Five Ways. While I greatly admire the five ways I do think that Aquinas was hasty in assuming that omnes dicunt Deum. Perhaps all in his time would have, but certainly not now. That is unfortunate but what can be done? I for my part think that many good arguments for the existence of God can be given.
Well, I am presently working my way slowly through Germain Grisez’s Beyond the New Theism: a philosophy of religion, and am enjoying it very much. He offers an argument for the existence of God that, while in the spirit of the Thomistic arguments, does not fall victim to many of the offhand criticisms directed towards cosmological arguments. I must say that I find it rather convincing, if not foolproof. It is compelling, but not necessarily knock-down. If one is looking for a knock-down argument then one will be looking a long time; there are always sophists willing to play the fool.
Here, in brief, is his argument in summary form (taken from pages 82-84). Be mindful that it is a modal argument of sorts, which is probably why I like it so much. It has its starting point, not in the nature of the whole universe itself or the apparent characteristic of design in nature, but rather from the mere fact that contingent states of affairs obtain, and that some particular state of affairs obtains, whatever it may be:
“1. Some contingent state of affairs obtains. Nothing in the argument depends upon the particular content of the state of affairs from which it begins. Thus the argument can be generalised; it proceeds from any contingent state of affairs which obtains. “Contingent” here does not mean dependent or transitory. A contingent state of affairs is one which might or might not obtain; its obtaining does not follow from its being the state of affairs which it is. The primary evidence that there are contingent states of affairs is that we can know what it would be like for many states of affairs to obtain without knowing whether they obtain.
2. It is reasonable to ask why any particular contingent state of affairs obtains and to expect an answer which would begin to provide an unconditional explanation. An unconditional explanation would explain why a contingent state of affairs obtains otherwise than by reference to other contingent states of affairs themselves assumed to obtain. Although it is not reasonable, according to the preceding argument, to expect an explanation of why contingent states of affairs are contingent, it is reasonable to expect an explanation of why they obtain, unless there are good reasons for not proceeding with the inquiry. Hume, Kant, and philosophers who more or less closely follow them argue that human knowledge is necessarily limited in certain ways which would rule out the question, the answer, or both. I argue… that such attempts to delimit knowledge fail.
3. The question why a contingent state of affairs obtains is not satisfactorily answered by positing a necessity which is identical with some or all contingent states of affairs. Hegel attempts to overcome the distinction between what contingent states of affairs are and their obtaining by his theory of Absolute Spirit. Post-hegelian relativism limits philosophic explanation to cases in which meaning originates in human action and thus coincides with a state of affairs which obtains. I argue… that such attempts to comprehend obtaining–that is, to explain it without an uncaused cause–fail.
4. The question why a contingent state of affairs obtains is not satisfactorily answered by saying that it obtains because it is the state of affairs which it is. A noncontingent state of affairs can be explained in this way. thus, if formal truths pick out state of affairs, it is reasonable to say that such states of affairs obtain–in the sense in which they do obtain–because they are what they are. It is also reasonable to say that an uncaused entity obtains simply because it is what it is. But since a contingent state of affairs is the state of affairs which it is whether it obtains or not, what a contingent state of affairs is cannot explain its obtaining.
5. The question why a contingent state of affairs obtains can and must be answered by saying that there is an uncaused entity, which necessarily obtains, and which causes contingent state of affairs to obtain. It must be admitted that in saying this one uses language in irregular ways. But an irregular use of language need not be arbitrary. If the way in which the irregular use is derived from ordinary uses can be clarified, extension and stretching of ordinary language and the bending of linguistic rules can be reasonable. I try to clarify the ways in which language is used in the conclusion of the argument and argue that the irregularity of these uses is reasonable.
6. To say that a contingent state of affairs is caused by an uncaused cause is only a partial answer to the question why it obtains. This answer is only partial for two reasons. No particular contingent state of affairs obtains in isolation. The relationship of contingent states of affairs to one another is part of the explanation of why they obtain. Philosophical and scientific explanations complement one another and in a sense bear upon the same subject matter, but they cannot substitute for one another. Moreover, the uncaused cause, Dc, and the uncaused entity which is its nucleus, D, emerge from the argument only as theoretical entities which are posited to satisfy the conditions of the problem. What is D in itself? How does Dc bring about contingent states of affairs? Such questions naturally arise, and it is false to suggest that one has a complete unconditional explanation of why contingent states of affairs obtain unless these questions are answered.”
That is the argument in rather brief form. I highly recommend the book, although it is very abstract and difficult analytic philosophy. It is one that is worth struggling through.