Philosophical proofs that God must be triune?

St. Thomas Aquinas, addressing the question of "Whether the trinity of the divine persons can be known by natural reason?" (Summa Theologica I q. 32 a. 1), considered it "impossible to attain to the knowledge of the Trinity by natural reason." He listed some objections to his opinion, in which theologians and philosophers tried to prove that God is triune:

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

1. 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

2. 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.

3. We read, moreover, in a gloss on Rm. 1 and Ex. 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

4. 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,

1. 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."

2. 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 (Question [27], Articles 1,3).

Therefore the trinity of persons can be known by natural reason.

Fr. Réginald Garrigou-Lagrange, O.P., commentating on this passage in ch. 6, "The Knowability of the Divine Persons," of his De Deo Trino, gives some examples of philosophers and theologians who, contrary to St. Thomas, did think it is possibility to know God is triune from natural reason:

State of the question. The question is well put by St. Thomas in the three difficulties proposed at the beginning of the article.

1. Many Platonic and Neoplatonic philosophers admitted a certain kind of Trinity with three hypostases, namely, the One, the Logos, and the world soul.
2. Richard of St. Victor tried to demonstrate the Trinity from the infinity of the divine goodness, which communicates itself infinitely in the procession of the three divine persons and from the fact that there can be no joyous possession of any good without some consort or association in that enjoyment. In a similar way, St. Augustine proceeded to show the Trinity of persons from the procession of the word and of love in our human minds.
3. If the mystery of the Trinity had no relation to our reason, its revelation would seem to be superfluous.

We might add that Abelard tried to demonstrate the Trinity.^[333] St. Anselm frequently attempted to construct demonstrations to prove the Trinity and sometimes indulged in what were at least wordy extravagances. In recent times Guenther also wished to demonstrate this mystery,^[334] as did Rosmini, who brought down on himself the Church's condemnation.^[335] More recently Schell, in opposition to the rationalists and Unitarians, who said this mystery was openly opposed to reason, tried to prove the Trinity from the nexus between aseity and immanent processions.^[336]

_NOTES

333. Theologia christiana, I, 5
334. Cf. Vacant, Etudes sur le Concile Vatican, I, 130
335. Denz., no. 1915
336. Pesch, Dogmatica, p. 274