The book is really excellent.
In his book
Reality (St. Louis, MO: B. Herder Book Co., 1958), Rev. Father Reginald Garrigou-Lagrange, commenting upon the treatise
De Deo uno as found in the
Summa of St. Thomas Aquinas wrote:
St. Thomas does not admit that an a priori proof of God's existence can be given [1a, q. 2, a. 1]. He grants indeed that the preposition, God exists, is in itself evident, and would therefore be self-evident to us if we had a priori face-to-face knowledge of God; then we would see that His essence includes existence, not merely as an object of abstract thought, but as a reality objectively present. But in point of fact we have no such a priori knowledge of God. We must begin with a nominal definition of God, conceiving Him only confusedly, as the first source of all that is real and good in the world. From this abstract knowledge, so far removed form direct intuition of God's essence, we cannot deduce a priori His existence as a concrete fact.
It is true we can know a priori the truth of this proposition: If God exists in fact, then He exists of Himself. But in order to know that He exists in fact, we must begin with existences which we know by sense experience, and then proceed to see if these concrete existences necessitate the actual objective existence of a First Cause, corresponding to our abstract concept, our nominal definition of God [1a, q. 2, a. 1, ad 2; a. 2, ad 2].
A little further on, he proceeds to discuss the five classical proofs proposed by St. Thomas regarding the existence of God:
The five classical proofs for God's existence rest, one and all, on the one principle of causality, expressed in ever deepening formulas, as follows. First: whatever begins has a cause. Second: every contingent thing, even if it should be ab aeterno, depends on a cause which exists of itself. Third: that which has a share in existence depends ultimately on a cause which is existence itself, a cause whose very nature is to exist, which alone can say: I am who am [Exod. ch. iii., 14].
[...]
Most simply expressed, causality means: the more does not come from the less, the more perfect cannot be produced from the less perfect. In the world we find things which reach existence and then disappear, things whose life is temporary and perishable, men whose wisdom or goodness or holiness is limited and imperfect; then above all this limited perfection we must find at the summit Him who from all eternity is self-existing perfection, who is life itself, wisdom itself, goodness itself, holiness itself.
He later writes something that fundamentally explains the nature of God:
In God alone are essence and existence identified. In this supreme principle lies the real and essential distinction of God from the world. This distinction reveals God as unchangeable and the world as changeable (the first three proofs for His existence). It becomes more precise when it reveals God as absolutely simple and the world as multifariously composed (the fourth and fifth proofs). It finds its definitive formula when it reveals God as "He who is," whereas all other things are only receivers of existence, hence composed as receiver and received, of essence and existence. The creature is not its own existence, it has existence after receiving it.
He then goes on to clarify the role of reason and common sense in this argument:
This truth is vaguely grasped by the common sense of natural reason, which, by a confused intuition, sees that the principle of identity is the supreme law of reality, and hence the supreme law of thought. As A is identified with A, so is supreme reality identified with absolutely one and immutable Being, transcendentally and objectively distinct from the universe, which is essentially diversified and mutable.
From what I understand, man can arrive at the certitude of God's existence only
a posteriori by observing the world around him through his senses and availing himself of his natural reason and common sense. Divine Revelation is the object of theology properly so called, the existence of God as naturally knowable to the human intellect being the object of the branch of philosophy known as Theodicy or Natural Theology.
In our day, we can glory in that the proofs of St. Thomas, far from being "debunked" by materialists and atheists, have been gloriously vindicated and immensely substantiated by the great progress of the empirical sciences. We know of sublime and beautiful things of this planet and of the universe that were unknown to our forefathers, and we are all the more blessed in that we have more whereupon to meditate and thereby adore the great God, saying "O Lord, our Lord, how wondrous is Thy Name in all the earth! For Thy magnificence is exalted above the heavens" (Ps. viii. 2), "For Thou, O Lord, hast gladdened me by Thy doing: and in the works of Thy hands will I exult. How have Thy works been magnified, O Lord! Thy thoughts are exceeding deep!" (Ps. xci. 5-6).