Objection 1. It seems that there is only one truth, according to which all things are true. For according to Augustine (De Trin. xv, 1), "nothing is greater than the mind of man, except God." Now truth is greater than the mind of man; otherwise the mind would be the judge of truth: whereas in fact it judges all things according to truth, and not according to its own measure. Therefore God alone is truth. Therefore there is no other truth but God.
Objection 2. Further, Anselm says (De Verit. xiv), that, "as is the relation of time to temporal things, so is that of truth to true things." But there is only one time for all temporal things. Therefore there is only one truth, by which all things are true.
On the contrary, it is written (Psalm 11:2), "Truths are decayed from among the children of men."
I answer that, In one sense truth, whereby all things are true, is one, and in another sense it is not. In proof of which we must consider that when anything is predicated of many things univocally, it is found in each of them according to its proper nature; as animal is found in each species of animal. But when anything is predicated of many things analogically, it is found in only one of them according to its proper nature, and from this one the rest are denominated. So healthiness is predicated of animal, of urine, and of medicine, not that health is only in the animal; but from the health of the animal, medicine is called healthy, in so far as it is the cause of health, and urine is called healthy, in so far as it indicates health. And although health is neither in medicine nor in urine, yet in either there is something whereby the one causes, and the other indicates health. Now we have said (1) that truth resides primarily in the intellect; and secondarily in things, according as they are related to the divine intellect. If therefore we speak of truth, as it exists in the intellect, according to its proper nature, then are there many truths in many created intellects; and even in one and the same intellect, according to the number of things known. Whence a gloss on Psalm 11:2, "Truths are decayed from among the children of men," says: "As from one man's face many likenesses are reflected in a mirror, so many truths are reflected from the one divine truth." But if we speak of truth as it is in things, then all things are true by one primary truth; to which each one is assimilated according to its own entity. And thus, although the essences or forms of things are many, yet the truth of the divine intellect is one, in conformity to which all things are said to be true.
Reply to Objection 1. The soul does not judge of things according to any kind of truth, but according to the primary truth, inasmuch as it is reflected in the soul, as in a mirror, by reason of the first principles of the understanding. It follows, therefore, that the primary truth is greater than the soul. And yet, even created truth, which resides in our intellect, is greater than the soul, not simply, but in a certain degree, in so far as it is its perfection; even as science may be said to be greater than the soul. Yet it is true that nothing subsisting is greater than the rational soul, except God.
Reply to Objection 2. The saying of Anselm is correct in so far as things are said to be true by their relation to the divine intellect.
Source: Summa Theologica (New Advent)