Objection 1. It would seem unfitting to distinguish three virtues of the speculative intellect, viz. wisdom, science and understanding. Because a species is a kind of science, as stated in Ethic. vi, 7. Therefore wisdom should not be condivided with science among the intellectual virtues.
Objection 2. Further, in differentiating powers, habits and acts in respect of their objects, we consider chiefly the formal aspect of these objects, as we have already explained (I, 77, 3). Therefore diversity of habits is taken, not from their material objects, but from the formal aspect of those objects. Now the principle of a demonstration is the formal aspect under which the conclusion is known. Therefore the understanding of principles should not be set down as a habit or virtue distinct from the knowledge of conclusions.
Objection 3. Further, an intellectual virtue is one which resides in the essentially rational faculty. Now even the speculative reason employs the dialectic syllogism for the sake of argument, just as it employs the demonstrative syllogism. Therefore as science, which is the result of a demonstrative syllogism, is set down as an intellectual virtue, so also should opinion be.
On the contrary, The Philosopher (Ethic. vi, 1) reckons these three alone as being intellectual virtues, viz. wisdom, science and understanding.
I answer that, As already stated (1), the virtues of the speculative intellect are those which perfect the speculative intellect for the consideration of truth: for this is its good work. Now a truth is subject to a twofold consideration--as known in itself, and as known through another. What is known in itself, is as a "principle," and is at once understood by the intellect: wherefore the habit that perfects the intellect for the consideration of such truth is called "understanding," which is the habit of principles.
On the other hand, a truth which is known through another, is understood by the intellect, not at once, but by means of the reason's inquiry, and is as a "term." This may happen in two ways: first, so that it is the last in some particular genus; secondly, so that it is the ultimate term of all human knowledge. And, since "things that are knowable last from our standpoint, are knowable first and chiefly in their nature" (Phys. i, text. 2, 3); hence that which is last with respect to all human knowledge, is that which is knowable first and chiefly in its nature. And about these is "wisdom," which considers the highest causes, as stated in Metaph. i, 1,2. Wherefore it rightly judges all things and sets them in order, because there can be no perfect and universal judgment that is not based on the first causes. But in regard to that which is last in this or that genus of knowable matter, it is "science" which perfects the intellect. Wherefore according to the different kinds of knowable matter, there are different habits of scientific knowledge; whereas there is but one wisdom.
Reply to Objection 1. Wisdom is a kind of science, in so far as it has that which is common to all the sciences; viz. to demonstrate conclusions from principles. But since it has something proper to itself above the other sciences, inasmuch as it judges of them all, not only as to their conclusions, but also as to their first principles, therefore it is a more perfect virtue than science.
Reply to Objection 2. When the formal aspect of the object is referred to a power or habit by one same act, there is no distinction of habit or power in respect of the formal aspect and of the material object: thus it belongs to the same power of sight to see both color, and light, which is the formal aspect under which color is seen, and is seen at the same time as the color. On the other hand, the principles of a demonstration can be considered apart, without the conclusion being considered at all. Again they can be considered together with the conclusions, since the conclusions can be deduced from them. Accordingly, to consider the principles in this second way, belongs to science, which considers the conclusions also: while to consider the principles in themselves belongs to understanding.
Consequently, if we consider the point aright, these three virtues are distinct, not as being on a par with one another, but in a certain order. The same is to be observed in potential wholes, wherein one part is more perfect than another; for instance, the rational soul is more perfect than the sensitive soul; and the sensitive, than the vegetal. For it is thus that science depends on understanding as on a virtue of higher degree: and both of these depend on wisdom, as obtaining the highest place, and containing beneath itself both understanding and science, by judging both of the conclusions of science, and of the principles on which they are based.
Reply to Objection 3. As stated above (55, A3,4), a virtuous habit has a fixed relation to good, and is nowise referable to evil. Now the good of the intellect is truth, and falsehood is its evil. Wherefore those habits alone are called intellectual virtues, whereby we tell the truth and never tell a falsehood. But opinion and suspicion can be about both truth and falsehood: and so, as stated in Ethic. vi, 3, they are not intellectual virtues.
Source: Summa Theologica (New Advent)