Objection 1. It seems that truth does not reside only in the intellect, but rather in things. For Augustine (Soliloq. ii, 5) condemns this definition of truth, "That is true which is seen"; since it would follow that stones hidden in the bosom of the earth would not be true stones, as they are not seen. He also condemns the following, "That is true which is as it appears to the knower, who is willing and able to know," for hence it would follow that nothing would be true, unless someone could know it. Therefore he defines truth thus: "That is true which is." It seems, then, that truth resides in things, and not in the intellect.
Objection 2. Further, whatever is true, is true by reason of truth. If, then, truth is only in the intellect, nothing will be true except in so far as it is understood. But this is the error of the ancient philosophers, who said that whatever seems to be true is so. Consequently mutual contradictories seem to be true as seen by different persons at the same time.
Objection 3. Further, "that, on account of which a thing is so, is itself more so," as is evident from the Philosopher (Poster. i). But it is from the fact that a thing is or is not, that our thought or word is true or false, as the Philosopher teaches (Praedicam. iii). Therefore truth resides rather in things than in the intellect.
On the contrary, The Philosopher says (Metaph. vi), " The true and the false reside not in things, but in the intellect."
I answer that, As the good denotes that towards which the appetite tends, so the true denotes that towards which the intellect tends. Now there is this difference between the appetite and the intellect, or any knowledge whatsoever, that knowledge is according as the thing known is in the knower, whilst appetite is according as the desirer tends towards the thing desired. Thus the term of the appetite, namely good, is in the object desirable, and the term of the intellect, namely true, is in the intellect itself. Now as good exists in a thing so far as that thing is related to the appetite--and hence the aspect of goodness passes on from the desirable thing to the appetite, in so far as the appetite is called good if its object is good; so, since the true is in the intellect in so far as it is conformed to the object understood, the aspect of the true must needs pass from the intellect to the object understood, so that also the thing understood is said to be true in so far as it has some relation to the intellect. Now a thing understood may be in relation to an intellect either essentially or accidentally. It is related essentially to an intellect on which it depends as regards its essence; but accidentally to an intellect by which it is knowable; even as we may say that a house is related essentially to the intellect of the architect, but accidentally to the intellect upon which it does not depend.
Now we do not judge of a thing by what is in it accidentally, but by what is in it essentially. Hence, everything is said to be true absolutely, in so far as it is related to the intellect from which it depends; and thus it is that artificial things are said to be true a being related to our intellect. For a house is said to be true that expresses the likeness of the form in the architect's mind; and words are said to be true so far as they are the signs of truth in the intellect. In the same way natural things are said to be true in so far as they express the likeness of the species that are in the divine mind. For a stone is called true, which possesses the nature proper to a stone, according to the preconception in the divine intellect. Thus, then, truth resides primarily in the intellect, and secondarily in things according as they are related to the intellect as their principle. Consequently there are various definitions of truth. Augustine says (De Vera Relig. xxxvi), "Truth is that whereby is made manifest that which is;" and Hilary says (De Trin. v) that "Truth makes being clear and evident" and this pertains to truth according as it is in the intellect. As to the truth of things in so far as they are related to the intellect, we have Augustine's definition (De Vera Relig. xxxvi), "Truth is a supreme likeness without any unlikeness to a principle": also Anselm's definition (De Verit. xii), "Truth is rightness, perceptible by the mind alone"; for that is right which is in accordance with the principle; also Avicenna's definition (Metaph. viii, 6), "The truth of each thing is a property of the essence which is immutably attached to it." The definition that "Truth is the equation of thought and thing" is applicable to it under either aspect.
Reply to Objection 1. Augustine is speaking about the truth of things, and excludes from the notion of this truth, relation to our intellect; for what is accidental is excluded from every definition.
Reply to Objection 2. The ancient philosophers held that the species of natural things did not proceed from any intellect, but were produced by chance. But as they saw that truth implies relation to intellect, they were compelled to base the truth of things on their relation to our intellect. From this, conclusions result that are inadmissible, and which the Philosopher refutes (Metaph. iv). Such, however, do not follow, if we say that the truth of things consists in their relation to the divine intellect.
Reply to Objection 3. Although the truth of our intellect is caused by the thing, yet it is not necessary that truth should be there primarily, any more than that health should be primarily in medicine, rather than in the animal: for the virtue of medicine, and not its health, is the cause of health, for here the agent is not univocal. In the same way, the being of the thing, not its truth, is the cause of truth in the intellect. Hence the Philosopher says that a thought or a word is true "from the fact that a thing is, not because a thing is true."
Source: Summa Theologica (New Advent)