As a certain gloss on the verse The true have vanished, etc., of the Psalms says, the one First Truth is reflected by many truths in our minds, just as one person's face is reflected by many faces in a broken mirror. Now, this manifold reflection of the one First Truth involves two things. One is the light of our intellect, of which the Psalms speak: The light of your face, O Lord is signed upon us. The other is the first principles (simple or compound) that we naturally know. For we are only able to know the truth because of these first principles and the light of our intellect, and they are only able to make the truth clear to us because they resemble the First Truth, which makes them to a certain extent unchangeable and incapable of misleading us.
[Thomas Aquinas, Quodlibetal Questions, QQ. 10, art. 1, Turner Nevitt and Brian Davies, trs., Oxford University Press (New York: 2020), p. 142.]
The verse mentioned in the first sentence is Psalm 12:1; due to numbering differences, the gloss is the gloss on Psalm 11:2. The verse mentioned in the third sentence is Psalm 4:6 (Psalm 4:7 in the Vulgate), one of St. Thomas's favorite verses. Simple first principles are objects of the intellect like being and good; compound first principles are things like the principle of noncontradiction or the first principle of practical reason. In a sense, just as we can call God 'Truth Itself', we can call God 'The Principle of Noncontradiction Itself', that actual First Truth of which our understanding of the principle of noncontradiction is the participation.
