Objection 1. It would seem that the reason is a distinct power from the intellect. For it is stated in De Spiritu et Anima that "when we wish to rise from lower things to higher, first the sense comes to our aid, then imagination, then reason, then the intellect." Therefore the reason is distinct from the intellect, as imagination is from sense.
Objection 2. Further, Boethius says (De Consol. iv, 6), that intellect is compared to reason, as eternity to time. But it does not belong to the same power to be in eternity and to be in time. Therefore reason and intellect are not the same power.
Objection 3. Further, man has intellect in common with the angels, and sense in common with the brutes. But reason, which is proper to man, whence he is called a rational animal, is a power distinct from sense. Therefore is it equally true to say that it is distinct from the intellect, which properly belongs to the angel: whence they are called intellectual.
On the contrary, Augustine says (Gen. ad lit. iii, 20) that "that in which man excels irrational animals is reason, or mind, or intelligence or whatever appropriate name we like to give it." Therefore, reason, intellect and mind are one power.
I answer that, Reason and intellect in man cannot be distinct powers. We shall understand this clearly if we consider their respective actions. For to understand is simply to apprehend intelligible truth: and to reason is to advance from one thing understood to another, so as to know an intelligible truth. And therefore angels, who according to their nature, possess perfect knowledge of intelligible truth, have no need to advance from one thing to another; but apprehend the truth simply and without mental discussion, as Dionysius says (Div. Nom. vii). But man arrives at the knowledge of intelligible truth by advancing from one thing to another; and therefore he is called rational. Reasoning, therefore, is compared to understanding, as movement is to rest, or acquisition to possession; of which one belongs to the perfect, the other to the imperfect. And since movement always proceeds from something immovable, and ends in something at rest; hence it is that human reasoning, by way of inquiry and discovery, advances from certain things simply understood--namely, the first principles; and, again, by way of judgment returns by analysis to first principles, in the light of which it examines what it has found. Now it is clear that rest and movement are not to be referred to different powers, but to one and the same, even in natural things: since by the same nature a thing is moved towards a certain place. Much more, therefore, by the same power do we understand and reason: and so it is clear that in man reason and intellect are the same power.
Reply to Objection 1. That enumeration is made according to the order of actions, not according to the distinction of powers. Moreover, that book is not of great authority.
Reply to Objection 2. The answer is clear from what we have said. For eternity is compared to time as immovable to movable. And thus Boethius compared the intellect to eternity, and reason to time.
Reply to Objection 3. Other animals are so much lower than man that they cannot attain to the knowledge of truth, which reason seeks. But man attains, although imperfectly, to the knowledge of intelligible truth, which angels know. Therefore in the angels the power of knowledge is not of a different genus fro that which is in the human reason, but is compared to it as the perfect to the imperfect.
Source: Summa Theologica (New Advent)