Fundamental Validity of the Five Ways
“All these proofs rest on the principle of causality: Anything that exists, if it does not exist of itself, depends in last analysis on something that does exist of itself. To deny this principle leads to absurdity. To say “a thing contingent, that is, a thing which of itself does not have existence, is nevertheless uncaused” is equivalent to saying: A thing may exist of itself and simultaneously not exist of itself. Existence of itself would belong to it, both necessary and impossibly. Existence would be an inseparable predicate of a being which can be separated from existence. All this is absurd, unintelligible. Kant here objects. It is absurd, he says, for human intelligence, but not perhaps in itself absurd and unintelligible.
In answer, let us define absurdity. Absurd is that which cannot exist because it is beyond the bounds of objective reality, without any possible relation to reality. It is agreement between two terms which objectively can never agree. Thus, an uncaused union of things in themselves diverse is absurd. The only cause of union is unity. Union means a share in unity, because it presupposes things which are diverse, brought together by a higher unity. When you say: ‘Anything (from angel to grain of sand) can arise without any cause from absolute nothing,’ then you are making a statement which is not merely unsupported and gratuitous, but which is objectively absurd. Hence, we repeat: A being which is not self-existent, which only participates in existence, presupposes necessarily a Being which by nature is self-existent. Unity by participation presupposes unity by essence.
We have here presented the principle of causality, as St. Thomas does in question three, by the way that ascends from effect to cause. The same truth can be treated in the descending order, from cause to effect, as it is in fact treated later in the Summa. Many modern authors proceed from this second viewpoint, But the first order ought to precede the second.
To proceed. The denial of the principle of causality is not, it is true, a contradiction is immediately evident as if I were to say: ‘The contingent is not contingent.’ St. Thomas gives reason why this is so. In denying causality, he says, we do not deny the definition itself of the contingent. What we do deny is, not the essence of contingent, but an immediate characteristic of that essence. But to deny the principle as thus explained is as absurd as to affirm that we cannot, knowing the essence of a thing, deduce from that essence its characteristics. Hence to deny essential dependence of contingent being on its cause leads to absurdity, because such denial involves the affirmation that existence belongs positively to a thing which is not by nature self-existent and still is uncaused. Thus we would have, in one subject, the presence both of unessential existence and of non-dependence on any cause of its existence: a proposition objectively absurd.
But we find the denial of this principle of causality in ways that are still less evidently contradictory (in Spinoza, for example) where the contradiction is, at first sight, hidden and unapparent. To illustrate. Some who read the sentence, ‘Things incorporeal can of themselves occupy a place,’ cannot at once see that the sentence contains a contradiction. And still it is absurd to think that a spirit, which lives in an order higher than the order of quantity and space, should nevertheless be conceived as of itself filling place, place being a consequence of quantity and space.
Likewise there are contradictions which emerge only under the light of revelation. Suppose, as illustration, a man says there are four persons in God. Faith, not reason, tells us the proposition is absurd. Only those who enjoy the beatific vision, who know what God is, can see the proposition’s intrinsic absurdity.
If denial or doubt of the principle of causality leads to doubt or denial of the principle of contradiction, then the five classic proofs, truly understood, of God’s existence cannot be rejected without finding absurdity at the root of all reality. We must choose: either the Being who exists necessarily and eternally, who alone can say ‘I am truth and life,’ or then a radical absurdity at the heart of the universe. If truly God is necessary Being, on which all else depends, then without Him the existence of anything else becomes impossible, inconceivable, absurd. In point of fact, those who will not admit the existence of a supreme and universal cause, which is itself existence, and life, must content themselves with a creative evolution, which, lacking any raison d’etre, becomes a contradiction: universal movement, without subject distinct from itself, without efficient cause distinct from itself, without a goal distinct from itself, an evolution wherein, without cause, the more arises from the less. Contradiction, identity, causality, all first principles go overboard. Let us repeat. Without a necessary and eternal being, on which all else depends, nothing exists and nothing can exist. To deny God’s existence and simultaneously to affirm any existence is to fall necessarily into contradiction, which does not always appear on the surface, in the immediate terms employed, but which is always there if you will but examine those terms. Many of Spinoza’s conclusions contain these absurdities. A fortiori, they lie hidden in atheistic doctrine which denies God’s existence. Hence, agnosticism, which doubts God’s existence, can thereby be led to doubt even the first principle of thought and reality, the principle of contradiction.
Having thus shown the validity of the five ways to prove God’s existence we now turn to dwell on their unity, the point where they all converge and culminate.”
– Lucas G. Westman
 Reality: A Synthesis of Thomistic Thought, Pg. 65-67