Objection 1. It would seem that one person cannot understand one and the same thing better than another can. For Augustine says (QQ. 83, qu. 32), "Whoever understands a thing otherwise than as it is, does not understand it at all. Hence it is clear that there is a perfect understanding, than which none other is more perfect: and therefore there are not infinite degrees of understanding a thing: nor can one person understand a thing better than another can."
Objection 2. Further, the intellect is true in its act of understanding. But truth, being a certain equality between thought and thing, is not subject to more or less; for a thing cannot be said to be more or less equal. Therefore a thing cannot be more or less understood.
Objection 3. Further, the intellect is the most formal of all that is in man. But different forms cause different species. Therefore if one man understands better than another, it would seem that they do not belong to the same species.
On the contrary, Experience shows that some understand more profoundly than do others; as one who carries a conclusion to its first principles and ultimate causes understands it better than the one who reduces it only to its proximate causes.
I answer that, A thing being understood more by one than by another may be taken in two senses.
First, so that the word "more" be taken as determining the act of understanding as regards the thing understood; and thus, one cannot understand the same thing more than another, because to understand it otherwise than as it is, either better or worse, would entail being deceived, and such a one would not understand it, as Augustine argues (QQ. 83, qu. 32). In another sense the word "more" can be taken as determining the act of understanding on the part of him who understands; and so one may understand the same thing better than someone else, through having a greater power of understanding: just as a man may see a thing better with his bodily sight, whose power is greater, and whose sight is more perfect. The same applies to the intellect in two ways.
First, as regards the intellect itself, which is more perfect. For it is plain that the better the disposition of a body, the better the soul allotted to it; which clearly appears in things of different species: and the reason thereof is that act and form are received into matter according to matter's capacity: thus because some men have bodies of better disposition, their souls have a greater power of understanding, wherefore it is said (De Anima ii, 9), that "it is to be observed that those who have soft flesh are of apt mind."
Secondly, this occurs in regard to the lower powers of which the intellect has need in its operation: for those in whom the imaginative, cogitative, and memorative powers are of better disposition, are better disposed to understand.
The reply to the First Objection is clear from the above; likewise the reply to the
Second, for the truth of the intellect consists in the intellect understanding a thing as it is.
Reply to Objection 3. The difference of form which is due only to the different disposition of matter, causes not a specific but only a numerical difference: for different individuals have different forms, diversified according to the difference of matter.
Source: Summa Theologica (New Advent)