Prima Pars, Question 32 — Summa Theologiae
Article 1. Whether the trinity of the divine persons can be known by natural reason?
Objection 1. It would seem that the trinity of the divine persons can be known by natural reason. For philosophers came to the knowledge of God not otherwise than by natural reason. Now we find that they said many things about the trinity of persons, for Aristotle says (De Coelo et Mundo i, 2): “Through this number”—namely, three—“we bring ourselves to acknowledge the greatness of one God, surpassing all things created.” And Augustine says (Confess. vii, 9): “I have read in their works, not in so many words, but enforced by many and various reasons, that in the beginning was the Word, and the Word was with God, and the Word was God,” and so on; in which passage the distinction of persons is laid down. We read, moreover, in a gloss on Romans 1 and Exodus 8 that the magicians of Pharaoh failed in the third sign—that is, as regards knowledge of a third person—i.e. of the Holy Ghost —and thus it is clear that they knew at least two persons. Likewise Trismegistus says: “The monad begot a monad, and reflected upon itself its own heat.” By which words the generation of the Son and procession of the Holy Ghost seem to be indicated. Therefore knowledge of the divine persons can be obtained by natural reason.
Objection 2. Further, Richard St. Victor says (De Trin. i, 4): “I believe without doubt that probable and even necessary arguments can be found for any explanation of the truth.” So even to prove the Trinity some have brought forward a reason from the infinite goodness of God, who communicates Himself infinitely in the procession of the divine persons; while some are moved by the consideration that “no good thing can be joyfully possessed without partnership.” Augustine proceeds (De Trin. x, 4; x, 11,12) to prove the trinity of persons by the procession of the word and of love in our own mind; and we have followed him in this (I:27:1 and I:27:3). Therefore the trinity of persons can be known by natural reason.
Objection 3. Further, it seems to be superfluous to teach what cannot be known by natural reason. But it ought not to be said that the divine tradition of the Trinity is superfluous. Therefore the trinity of persons can be known by natural reason.
On the contrary, Hilary says (De Trin. i), “Let no man think to reach the sacred mystery of generation by his own mind.” And Ambrose says (De Fide ii, 5), “It is impossible to know the secret of generation. The mind fails, the voice is silent.” But the trinity of the divine persons is distinguished by origin of generation and procession (I:30:2. Since, therefore, man cannot know, and with his understanding grasp that for which no necessary reason can be given, it follows that the trinity of persons cannot be known by reason.
I answer that, It is impossible to attain to the knowledge of the Trinity by natural reason. For, as above explained (I:12:4 and I:12:12), man cannot obtain the knowledge of God by natural reason except from creatures. Now creatures lead us to the knowledge of God, as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to Him as the principle of things, and we have cited this fundamental principle in treating of God as above (I:12:12). Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the persons. Therefore, by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the persons. Whoever, then, tries to prove the trinity of persons by natural reason, derogates from faith in two ways.
Firstly, as regards the dignity of faith itself, which consists in its being concerned with invisible things, that exceed human reason; wherefore the Apostle says that “faith is of things that appear not” (Hebrews 11:1), and the same Apostle says also, “We speak wisdom among the perfect, but not the wisdom of this world, nor of the princes of this world; but we speak the wisdom of God in a mystery which is hidden” (1 Corinthians 2:6-7).
Secondly, as regards the utility of drawing others to the faith. For when anyone in the endeavor to prove the faith brings forward reasons which are not cogent, he falls under the ridicule of the unbelievers: since they suppose that we stand upon such reasons, and that we believe on such grounds.
Therefore, we must not attempt to prove what is of faith, except by authority alone, to those who receive the authority; while as regards others it suffices to prove that what faith teaches is not impossible. Hence it is said by Dionysius (Div. Nom. ii): “Whoever wholly resists the word, is far off from our philosophy; whereas if he regards the truth of the word”—i.e. “the sacred word, we too follow this rule.”