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On the Nature of God
We have seen how the classical proofs for the existence of God as presented by St. Thomas demonstrate the existence of a first mover of spiritual and corporeal beings, of a first cause of everything that comes into existence, of a necessary being on which all contingent and perishable things depend, of a supreme being, the first truth and sovereign good, and of an intelligent designer, the cause of order in the universe, to which we rightly give the name providence.
Now it is through these five attributes (first mover, first cause, etc.) that we have our conception of God. We have thus proved His existence. We must now go on to state what He is, what formally constitutes His nature. We cannot otherwise form a right idea of providence.
Here on earth, of course, we can have no knowledge of the divine essence as it is really in itself; for this we must have an intuitive vision of it as the blessed see it in heaven. Our knowledge of God here on earth is obtained solely through the reflection of His perfections in the mirror of created things. Since these are on a plane far inferior to His, they do not enable us to know Him as He is in Himself. As Plato tells us in his allegory of the cave, where God is concerned we are to some extent like men who have never seen the sun but simply a reflection of its rays in the things it illuminates; or like men who have never seen white light but only the seven colors of the rainbow: violet, indigo, blue, green, yellow, orange, and red. For such men a right conception of white light would be impossible; they could have only a negative or relative conception of it as an inaccessible source of light. It is the same with the Divine nature: we cannot form a proper and positive conception of it through creatures, for the perfections which in God form an absolute unity are in creatures multiple and divided.
Here on earth, therefore, it is impossible for us to know the Divine nature as it is in itself. If this were possible, we should see how all the divine perfections contained in it --- such as infinite being, wisdom, love, justice, mercy --- are really identified, yet without destroying one another. As it is, we are reduced to spelling out, as it were, and enumerating these Divine perfections one after another, always with the reservation that they are identified in one transcendent simplicity, in the higher unity of the Deity or Divinity. But the Deity or the very essence. of God --- that which makes God to be God --- we do not see, nor shall we ever be able to do so until we reach Heaven. It is as though we were gazing at the sides of a pyramid the summit of which remains ever invisible.
But, without knowing the Divine nature as it is in itself, can we not determine, so far as our imperfect mode of knowing permits, what it is that formally constitutes that nature? In other words, among all the perfections we attribute to God is there not one that is fundamental, the source as it were of all the Divine attributes and likewise the principle distinguishing God from the world?
Is there not in God some radical perfection having the same function in Him as rationality in man? Man is defined as a rational being; this, distinguishing him from inferior beings, is the principle of his distinctive human characteristics. Because man is rational, he is free, he is morally responsible for his actions, he is social and religious, he has the faculty of speech and intelligent laughter. These characteristics do not exist in the brute beast. We deduce man's characteristics as we. deduce the properties of the triangle or the circle.
Is there in God some radical perfection also that allows of our defining Him, according to our imperfect mode of knowledge, in some such way as we define man, or again as we define a circle or a pyramid? In other words, is there not a certain order in the Divine perfections, so that from one primary perfection all the rest may be deduced? This is the statement of the problem.
The various solutions
To the question thus stated various solutions have been given. Beginning with the least satisfactory, we shall proceed by degrees to the most profound.
1) Some (Nominalists) have held that in God there is no fundamental perfection from which the rest may be logically deduced. According to their view, the Divine essence is merely the sum of all the perfections; there can be no question of seeking a logical order among them, since they are simply different names for the same transcendent reality.
This doctrine of Nominalism leads to the conclusion that God is unknowable, because His attributes cannot be deduced from one fundamental Divine perfection; and, since we can give no reason why He must be wise or just or merciful, we should simply be asserting the fact without knowing why.
2) Others, inspired by Descartes, have held that what constitutes the Divine nature is liberty: God is pre-eminently a will transcendently free. Descartes claimed that, if God so willed, He could make the circle square, mountains without valleys, or beings that at one and the same time would exist and not exist, or effects without a cause. Ockham in the Middle Ages declared that, had God so willed, He could have commanded us not to love but to hate both ourselves and Him. That is, the principle of contradiction and the distinction between moral good and evil are dependent for their truth on the free will of God. First and foremost God is said to be absolute liberty.
In the opinion of some modern philosophers (Secrétan in Switzerland, for instance), the correct definition of God is I am what I will, I am what I would freely be.
In reply to this view, it has been pointed out that liberty cannot be conceived as anterior to intellect. Liberty without intellect is impossible; it would be confounded with mere chance. Liberty is inconceivable without an intellect to direct it; it would be liberty without standard of any kind, without truth, without true goodness. As Leibniz remarked, to say that God, if He had wished, could have commanded us to hate Him, ia to deny that He is of necessity the sovereign good; in that case, had He wished, He might well have been the Manichean principle of evil. A man would have be out of his senses to maintain such a position. To claim that God has established the distinction between good and evil by a purely arbitrary decree, to claim that He is absolute liberty without standard of any kind, is, as Leibniz again says, "to dishonor God."
Clearly, then, liberty cannot be conceived without an intellect and wisdom to direct it, and conversely intellect is conceived as anterior to the liberty it directs. The knowledge of true good, indeed, is anterior to the love of that good, which would not be so loved were it not already known.
Intellect, therefore, is prior to and the cause of liberty. Shall we say, then, that what formally constitutes the Divine nature is intellect, the ever actual thought or eternal knowledge of the true in all its fulness? This, of course, is a Divine perfection, but is it the fundamental perfection?
A number of philosophers and theologians thought so. They conceived of God as pre-eminently a pure intellectual flash subsisting eternally. During a storm at night, an immense streak of lightning may sometimes be seen, flashing from one extremity of the sky to the other; this, they would say, is a faint image of God. We also speak of "flashes of genius," as in the case of Newton's discovery of the great laws of nature. These are transitory and very confined flashes, revealing what after all is only a partial truth, like the law of universal gravitation. God, on the other hand, is a pure intellectual flash subsisting eternally, who is infinite truth and sees in one glance all actual and possible worlds, with all their laws. God is, indeed, eternally subsistent thought itself, truth itself ever actually known. And why is this? Because intellectual life is the highest form of life, transcending vegetative plant life and sensitive animal life; because, too, intellect is anterior to will and liberty, which it directs by pointing out the good to be desired and loved.
This is all quite true. But is subsistent thought or intellection the absolutely primary perfection in God? However lofty this way of conceiving the Divine nature may be, it does not seem to be the highest. 
Holy Scripture provides us with a more profound conception of the Divine nature. It tells us that God is being itself; He Himself has revealed His name to us as "He Who is."
God is the eternally subsisting being
In the Book of Exodus 3:14), we are told how God, speaking to Moses from the burning bush, revealed His name. He did not say, "I am absolute liberty, I am what I will"; nor did He say, "I am intellect itself, thought eternally subsistent." He said, "I am Who am," that is, the eternally subsistent being.
Let us call to mind this passage from Exodus: "Moses said to God: Lo, I shall go to the children of Israel, and Say to them: The God of your fathers hath sent me to you. If they should say to me: What is His name? what shall I say to them? God said to Moses: I am Who am. He said: Thus shalt thou say to the children of Israel: He Who is hath sent me to you." He Who is: in Hebrew, Yahweh, from which the word Jehovah has been formed. "This is my name forever, and this is my memorial unto all generations" (Ibid., 15).
Again, in the last book of the New Testament (Apocalypse, 1:8), we read: "I am Alpha and Omega, the beginning and the end, saith the Lord God, Who is and Who was and Who is to come, the Almighty." (Cf. 1:4.)
Under this title God has frequently revealed Himself to His Saints, to St. Catherine of Siena, for instance: "I am He Who is, thou art that which is not."
God, then, is not only pure spirit, He is being itself subsisting immaterial at the summit of all things and transcending any limits imposed by either space or matter or a finite spiritual essence.
In our imperfect mode of knowledge, must we not say that subsistent being is the formal constituent of the Divine nature?
It would not seem a difficult matter to establish the truth of this. In fact, what formally constitutes the Divine nature is that which in God we conceive to be the fundamental perfection distinguishing Him from creatures and the source from which His attributes are deduced.
Now, because God is the self-subsisting being, the infinite ocean of spiritual being, unlimited, un materialized, He is distinguished from every material or spiritual creature. The Divine essence alone is existence itself, it alone of necessity exists. No creature is self-existent; none can say: I am being, truth, life, etc. Jesus alone among men said, "I am the truth and the life," which was equivalent to saying, "I am God."
Upon this culminating point, namely, the self-subsisting being, converge the five proofs for the existence of God, as developed by St. Thomas: the first mover, the first cause, the necessary being, the supreme being, the intelligent designer of order in the universe. All these attributes must be predicated of the self-subsisting and immaterial being who is at the summit of all things. Again, from this culminating point are deduced all the Divine attributes, as the characteristics of man are deduced from his rationality.
As will be seen more clearly in what follows, the self-subsisting and immaterial being who is at the summit of all things must be absolutely one and simple, must be truth itself ever actually known, the good itself ever actually loved. By reason of His perfect and unique immateriality He must be intelligence itself, thought itself eternally subsistent, wisdom itself; subsistent will and love; hence justice and mercy.
Conversely, we see that justice and mercy presuppose the love of the good; that love presupposes an intellect which enlightens it; that intellect presupposes an intelligent being and at the same time an intelligible being which it contemplates.
It remains true, therefore, that of all the names of God, the primary and most distinctive is "He Who is," Yahweh. It is pre-eminently His name, says St. Thomas (Ia, q. 13, a. 11), and that for three reasons:
I) Because it expresses not one form of being or one particular essence, but being itself; and God alone is being itself, He alone is self-existent.
2) It is the most universal name, embracing being in all its fulness, with all its perfections --- the boundless, shoreless, ocean, as it were, of omnipotent, omniscient, spiritual substance.
3) This name, "He Who is," signifies not only being, but the ever-present being, for whom there is neither past nor future.
Here, then, is what formally constitutes the Divine nature according to our imperfect manner of understanding, which consists in deducing from this formal constituent the Divine attributes, enumerating them one after another: unity, wisdom, love, justice, mercy and the rest, yet without ever perceiving how they are fused together and identified in the intimate life of God, which is the Deity.
In this life we can have no knowledge of the Deity, of the Divine nature, such as it really is; for this we should need to have an intuitive vision of it as the blessed have in Heaven, without the intervention of any created image. Only in heaven shall we see how wisdom is identical with God's utterly free good pleasure; how, for all its freedom, this good pleasure is by no means a caprice, since it is penetrated through and through by wisdom. Then only shall we see how infinite justice and mercy are identified in the love of the sovereign good, which has the right to be loved above all else and which tends to communicate itself to us for our happiness.
The Deity, as it really is, remains for us a secret, a profound mystery. Indeed, the mystics have called it the Great Darkness, a light-transcending darkness; it is the "light inaccessible" spoken of in Scripture.
Although we cannot have knowledge of the Deity as it really is, we are permitted to participate in it: through sanctifying grace, which is in very truth a participation in the Divine nature as it really is,  preparing us in this present life to see and love God some day as He sees and loves Himself. From this we see the value of sanctifying grace, which far surpasses the natural life of the intellect, whether in us or even in the Angels. This truth leads St. Thomas to remark that the least degree of sanctifying grace in the soul of a little child just Baptized is of more value than all corporeal and spiritual natures taken together: "The good of grace in one is greater than the good of nature in the whole universe" (Ia IIae, q. 113, a. 9 ad 2um).
Pascal expresses this well in one of the finest pages of his Pensées: "The least of minds is greater than all material objects, the firmament, the stars, the earth and its kingdoms; for the mind has knowledge of all these things and of itself; whereas things material have no knowledge at all. Bodies and minds, all these taken together and the effects produced by them, do not equal the least act of charity. This latter is of an infinitely higher order. From the sum total of material things there could not possibly issue one little thought, because thought is of another order. From bodies and minds we cannot possibly have an act of true charity, for the latter, too, is of another order, pertaining to the supernatural. The saints have their realm, their glory, their luster, and have no need of temporal or spiritual aggrandizement, which in no way affects them, neither increasing nor decreasing their greatness. The Saints are seen by God and the Angels, not by bodies or by curious minds. God suffices for them."  This sums up the value of the hidden life.
In the present life this holiness reveals most clearly, though in the obscurity of faith, what constitutes the intimate life of God, the Deity. This it does because holiness, which is the life of grace in its perfection, is a real, living participation in this same intimate life of God, preparing us to behold it some future day. Hence those words of the psalmist (Ps. 67:36): "God is wonderful in His Saints."
1. In favor of this view it is said that since sanctifying grace is a participation in the Divine nature ordered essentially to the beatific vision, it is a participation in that nature in so far as it is intellectual life. It would seem, then, that the Divine nature is fundamentally the supreme intellectual life, eternally subsistent thought, rather than being itself.
To this we reply that sanctifying grace is a participation in the Divine nature as it is in itself and not simply as our imperfect mode of knowledge conceives it. It is a participation in the Deity, whose formal signification transcends even that of being and intellection. Conceived simply as subsistent being, God contains only implicitly, actu implicite, the rest of the Divine perfections deducible from it; whereas the Deity as it is in itself and as contemplated by the blessed in Heaven contains all the Divine attributes explicitly, actu explicite. The blessed behold them immediately in the Deity and have no need to deduce them.
2. See preceding note.
3. Pascal, Pensées (Havet ed.), art. 18.