Reply Obj. 4: Because our intellect is not eternal, neither is the truth of enunciable propositions which are formed by us, eternal, but it had a beginning in time. Now before such truth existed, it was not true to say that such a truth did exist, except by reason of the divine intellect, wherein alone truth is eternal. But it is true now to say that that truth did not then exist: and this is true only by reason of the truth that is now in our intellect; and not by reason of any truth in the things. For this is truth concerning not-being; and not-being has not truth of itself, but only so far as our intellect apprehends it. Hence it is true to say that truth did not exist, in so far as we apprehend its not-being as preceding its being. _______________________
EIGHTH ARTICLE [I, Q. 16, Art. 8]
Whether Truth Is Immutable?
Objection 1: It seems that truth is immutable. For Augustine says (De Lib. Arbit. ii, 12), that "Truth and mind do not rank as equals, otherwise truth would be mutable, as the mind is."
Obj. 2: Further, what remains after every change is immutable; as primary matter is unbegotten and incorruptible, since it remains after all generation and corruption. But truth remains after all change; for after every change it is true to say that a thing is, or is not. Therefore truth is immutable.
Obj. 3: Further, if the truth of an enunciation changes, it changes mostly with the changing of the thing. But it does not thus change. For truth, according to Anselm (De Verit. viii), "is a certain rightness" in so far as a thing answers to that which is in the divine mind concerning it. But this proposition that "Socrates sits", receives from the divine mind the signification that Socrates does sit; and it has the same signification even though he does not sit. Therefore the truth of the proposition in no way changes.
Obj. 4: Further, where there is the same cause, there is the same effect. But the same thing is the cause of the truth of the three propositions, "Socrates sits, will sit, sat." Therefore the truth of each is the same. But one or other of these must be the true one. Therefore the truth of these propositions remains immutable; and for the same reason that of any other.
On the contrary, It is written (Ps. 11:2), "Truths are decayed from among the children of men."
I answer that, Truth, properly speaking, resides only in the intellect, as said before (A. 1); but things are called true in virtue of the truth residing in an intellect. Hence the mutability of truth must be regarded from the point of view of the intellect, the truth of which consists in its conformity to the thing understood. Now this conformity may vary in two ways, even as any other likeness, through change in one of the two extremes. Hence in one way truth varies on the part of the intellect, from the fact that a change of opinion occurs about a thing which in itself has not changed, and in another way, when the thing is changed, but not the opinion; and in either way there can be a change from true to false. If, then, there is an intellect wherein there can be no alternation of opinions, and the knowledge of which nothing can escape, in this is immutable truth. Now such is the divine intellect, as is clear from what has been said before (Q. 14, A. 15). Hence the truth of the divine intellect is immutable. But the truth of our intellect is mutable; not because it is itself the subject of change, but in so far as our intellect changes from truth to falsity, for thus forms may be called mutable. Whereas the truth of the divine intellect is that according to which natural things are said to be true, and this is altogether immutable.
Reply Obj. 1: Augustine is speaking of divine truth.