Objection 1. It seems that truth does not reside only in the intellect composing and dividing. For the Philosopher says (De Anima iii) that as the senses are always true as regards their proper sensible objects, so is the intellect as regards "what a thing is." Now composition and division are neither in the senses nor in the intellect knowing "what a thing is." Therefore truth does not reside only in the intellect composing and dividing.
Objection 2. Further, Isaac says in his book On Definitions that truth is the equation of thought and thing. Now just as the intellect with regard to complex things can be equated to things, so also with regard to simple things; and this is true also of sense apprehending a thing as it is. Therefore truth does not reside only in the intellect composing and dividing.
On the contrary, the Philosopher says (Metaph. vi) that with regard to simple things and "what a thing is," truth is "found neither in the intellect nor in things."
I answer that, As stated before, truth resides, in its primary aspect, in the intellect. Now since everything is true according as it has the form proper to its nature, the intellect, in so far as it is knowing, must be true, so far as it has the likeness of the thing known, this being its form, as knowing. For this reason truth is defined by the conformity of intellect and thing; and hence to know this conformity is to know truth. But in no way can sense know this. For although sight has the likeness of a visible thing, yet it does not know the comparison which exists between the thing seen and that which itself apprehends concerning it. But the intellect can know its own conformity with the intelligible thing; yet it does not apprehend it by knowing of a thing "what a thing is." When, however, it judges that a thing corresponds to the form which it apprehends about that thing, then first it knows and expresses truth. This it does by composing and dividing: for in every proposition it either applies to, or removes from the thing signified by the subject, some form signified by the predicate: and this clearly shows that the sense is true of any thing, as is also the intellect, when it knows "what a thing is"; but it does not thereby know or affirm truth. This is in like manner the case with complex or non-complex words. Truth therefore may be in the senses, or in the intellect knowing "what a thing is," as in anything that is true; yet not as the thing known in the knower, which is implied by the word "truth"; for the perfection of the intellect is truth as known. Therefore, properly speaking, truth resides in the intellect composing and dividing; and not in the senses; nor in the intellect knowing "what a thing is."
And thus the Objections given are solved.
Source: Summa Theologica (New Advent)