Objection 1. It would seem that the intellectual memory is distinct from the intellect. For Augustine (De Trin. x, 11) assigns to the soul memory, understanding, and will. But it is clear that the memory is a distinct power from the will. Therefore it is also distinct from the intellect.
Objection 2. Further, the reason of distinction among the powers in the sensitive part is the same as in the intellectual part. But memory in the sensitive part is distinct from sense, as we have said (78, 4). Therefore memory in the intellectual part is distinct from the intellect.
Objection 3. Further, according to Augustine (De Trin. x, 11; xi, 7), memory, understanding, and will are equal to one another, and one flows from the other. But this could not be if memory and intellect were the same power. Therefore they are not the same power.
On the contrary, From its nature the memory is the treasury or storehouse of species. But the Philosopher (De Anima iii) attributes this to the intellect, as we have said (6, ad 1). Therefore the memory is not another power from the intellect.
I answer that, As has been said above (Question 77, Article 3), the powers of the soul are distinguished by the different formal aspects of their objects: since each power is defined in reference to that thing to which it is directed and which is its object. It has also been said above (Question 59, Article 4) that if any power by its nature be directed to an object according to the common ratio of the object, that power will not be differentiated according to the individual differences of that object: just as the power of sight, which regards its object under the common ratio of color, is not differentiated by differences of black and white. Now, the intellect regards its object under the common ratio of being: since the passive intellect is that "in which all are in potentiality." Wherefore the passive intellect is not differentiated by any difference of being. Nevertheless there is a distinction between the power of the active intellect and of the passive intellect: because as regards the same object, the active power which makes the object to be in act must be distinct from the passive power, which is moved by the object existing in act. Thus the active power is compared to its object as a being in act is to a being in potentiality; whereas the passive power, on the contrary, is compared to its object as being in potentiality is to a being in act. Therefore there can be no other difference of powers in the intellect, but that of passive and active. Wherefore it is clear that memory is not a distinct power from the intellect: for it belongs to the nature of a passive power to retain as well as to receive.
Reply to Objection 1. Although it is said (3 Sent. D, 1) that memory, intellect, and will are three powers, this is not in accordance with the meaning of Augustine, who says expressly (De Trin. xiv) that "if we take memory, intelligence, and will as always present in the soul, whether we actually attend to them or not, they seem to pertain to the memory only. And by intelligence I mean that by which we understand when actually thinking; and by will I mean that love or affection which unites the child and its parent." Wherefore it is clear that Augustine does not take the above three for three powers; but by memory he understands the soul's habit of retention; by intelligence, the act of the intellect; and by will, the act of the will.
Reply to Objection 2. Past and present may differentiate the sensitive powers, but not the intellectual powers, for the reason give above.
Reply to Objection 3. Intelligence arises from memory, as act from habit; and in this way it is equal to it, but not as a power to a power.
Source: Summa Theologica (New Advent)