Commentaria in Summam contra Gentiles, Liber I

Second, Saint Thomas compares the first truth to natural reason¹. On this point he does two things: first, he shows what human reason can do with respect to the things that belong to faith; second, what is useful².

I. First, then, he says that in order to know the truth of faith, which can be most fully known only to those who see the divine substance, human reason can gather probable arguments, but not demonstrative ones. — This is proved. Sensible things, from which human reason takes the principle of its knowledge, bear within themselves some trace of the imitation of God, but not so perfect a one as to make manifest God's substance. Therefore, in order to know those things we believe about God, and which are most fully known only to those who see the divine substance, reason can indeed have probabilities, but not demonstrations.

The antecedent is clear for the first part: because they are effects of God; and an effect has some likeness to its cause. — The second part is also evident from what was said above³: namely, that they are weak and inadequate effects. — Likewise the consequence is known: because an effect, according to its condition, leads to knowledge of its cause.

II. It must be noted, for the explanation of the conclusion, that although faith has certainty in itself, that certainty is accompanied by inevidence: because the things that belong to faith neither appear to be true from the reasons of the terms, nor are they manifest from their resolution to the first principles naturally known, since they are above human reason; but we believe those things to be true only because they have been revealed by God, who is infallible truth. On account of this Saint Thomas says that only for those seeing the divine essence can they be most evidently known. For those who see the divine substance see that the things believed by us concerning the divinity are necessary, because from the reasons of the terms they know that the predicate inheres necessarily in the subject; which relation we cannot now see.

I think that what is said—that human reason can have plausible arguments—must be understood after the faith has been revealed, not before, since the human mind cannot raise itself of its own power to the things that are of faith. Hence Saint Thomas says in Iª, qu. XXXII, a. 1, ad 2, that reason can be brought in to make manifest the Trinity in a way that accords with a Trinity already posited, but not so as to prove sufficiently the trinity of Persons.

It must also be noted that Saint Thomas posits a twofold mode of evidence in these words: which, however, do not suffice for the aforementioned truth to be grasped as it were demonstratively or understood per se. For something becomes evident to the intellect in two ways: namely, either through necessary deduction from principles known per se, and this seems to occur through demonstration; or from the rationes of the terms, and this is said to be known per se to the intellect. Yet both draw their origin from sensible things: because the universal propositions from which demonstration is constituted are known only through experience, as is stated in Book I of the Metaphysics ⁴; likewise the rationes of the terms are deduced from sensible things, as the Philosopher teaches in Book II of the Posterior Analytics ⁵.

III. Secondly, in ⁶, he shows that, however weak such reasonings may be, it is nevertheless useful for the human mind to exercise itself in them: provided only that there be no presumption of demonstrating or comprehending. — This is proved by the authority of Aristotle adduced above: namely, because it is most delightful to be able to gaze upon the highest things even by a small and weak reasoning. — It is proved also by the authority of Hilary in his book On the Trinity, where he says: Begin to run through these things by believing.