The following is taken from St. Thomas' Summa Theologia, Ia, q. 32, a.1.
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It is impossible to attain to the knowledge of the Trinity by natural reason. For, as above explained (12, 4, 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 (Question 12, Article 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."
I love ad 2, where he distinguishes the two ways in which reason is employed:
ReplyDelete(1) for giving a "sufficient proof of some principle" (demonstration propter quid or from causes/principles to effects/conclusions)
or
(2) for "confirming an already established principle, by showing the congruity of its results" (demonstration quia, a posteriori, or from effects/conclusions to causes/principles).
Way #1 is how, e.g., Euclid's geometric proofs work (axioms → theorems); this is demonstration proper.
Way #2 is how, e.g., astronomy works (observations → theories), where multiple theories—e.g., the heliocentric or Ptolomaic theories—can "save the appearances" reported by observations.
Here's the relevant part of ad 2:
Reason may be employed in two ways to establish a point: firstly, for the purpose of furnishing sufficient proof of some principle, as in natural science [i.e., natural philosophy], where sufficient proof can be brought to show … [e.g., that a solar eclipse is caused by the moon blocking the sun for an observer on earth]. Reason is employed in another way, not as furnishing a sufficient proof of a principle, but as confirming an already established principle, by showing the congruity of its results, as in astrology [i.e., astronomy] the theory of eccentrics and epicycles is considered as established, because thereby the sensible appearances of the heavenly movements can be explained; not, however, as if this proof were sufficient, forasmuch as some other theory [e.g., Copernicus's heliocentric theory or Einstein's theory of gravity] might explain them. In the first way, we can prove that God is one; and the like. In the second way, reasons avail to prove the Trinity; as, when assumed to be true, such reasons confirm it. We must not, however, think that the trinity of persons is adequately proved by such reasons. …
Here, St. Thomas is laying the foundations for the modern scientific method, which employs reason in the sense of #2.