Objection 1. It would seem that memory is not in the intellectual part of the soul. For Augustine says (De Trin. xii, 2,3,8) that to the higher part of the soul belongs those things which are not "common to man and beast." But memory is common to man and beast, for he says (De Trin. xii, 2,3,8) that "beasts can sense corporeal things through the senses of the body, and commit them to memory." Therefore memory does not belong to the intellectual part of the soul.
Objection 2. Further, memory is of the past. But the past is said of something with regard to a fixed time. Memory, therefore, knows a thing under a condition of a fixed time; which involves knowledge under the conditions of "here" and "now." But this is not the province of the intellect, but of the sense. Therefore memory is not in the intellectual part, but only in the sensitive.
Objection 3. Further, in the memory are preserved the species of those things of which we are not actually thinking. But this cannot happen in the intellect, because the intellect is reduced to act by the fact that the intelligible species are received into it. Now the intellect in act implies understanding in act; and therefore the intellect actually understands all things of which it has the species. Therefore the memory is not in the intellectual part.
On the contrary, Augustine says (De Trin. x, 11) that "memory, understanding, and will are one mind."
I answer that, Since it is of the nature of the memory to preserve the species of those things which are not actually apprehended, we must first of all consider whether the intelligible species can thus be preserved in the intellect: because Avicenna held that this was impossible. For he admitted that this could happen in the sensitive part, as to some powers, inasmuch as they are acts of corporeal organs, in which certain species may be preserved apart from actual apprehension. But in the intellect, which has no corporeal organ, nothing but what is intelligible exists. Wherefore every thing of which the likeness exists in the intellect must be actually understood. Thus, therefore, according to him, as soon as we cease to understand something actually, the species of that thing ceases to be in our intellect, and if we wish to understand that thing anew, we must turn to the active intellect, which he held to be a separate substance, in order that the intelligible species may thence flow again into our passive intellect. And from the practice and habit of turning to the active intellect there is formed, according to him, a certain aptitude in the passive intellect for turning to the active intellect; which aptitude he calls the habit of knowledge. According, therefore, to this supposition, nothing is preserved in the intellectual part that is not actually understood: wherefore it would not be possible to admit memory in the intellectual part.
But this opinion is clearly opposed to the teaching of Aristotle. For he says (De Anima iii, 4) that, when the passive intellect "is identified with each thing as knowing it, it is said to be in act," and that "this happens when it can operate of itself. And, even then, it is in potentiality, but not in the same way as before learning and discovering." Now, the passive intellect is said to be each thing, inasmuch as it receives the intelligible species of each thing. To the fact, therefore, that it receives the species of intelligible things it owes its being able to operate when it wills, but not so that it be always operating: for even then is it in potentiality in a certain sense, though otherwise than before the act of understanding--namely, in the sense that whoever has habitual knowledge is in potentiality to actual consideration.
The foregoing opinion is also opposed to reason. For what is received into something is received according to the conditions of the recipient. But the intellect is of a more stable nature, and is more immovable than corporeal nature. If, therefore, corporeal matter holds the forms which it receives, not only while it actually does something through them, but also after ceasing to act through them, much more cogent reason is there for the intellect to receive the species unchangeably and lastingly, whether it receive them from things sensible, or derive them from some superior intellect. Thus, therefore, if we take memory only for the power of retaining species, we must say that it is in the intellectual part. But if in the notion of memory we include its object as something past, then the memory is not in the intellectual, but only in the sensitive part, which apprehends individual things. For past, as past, since it signifies being under a condition of fixed time, is something individual.
Reply to Objection 1. Memory, if considered as retentive of species, is not common to us and other animals. For species are not retained in the sensitive part of the soul only, but rather in the body and soul united: since the memorative power is the act of some organ. But the intellect in itself is retentive of species, without the association of any corporeal organ. Wherefore the Philosopher says (De Anima iii, 4) that "the soul is the seat of the species, not the whole soul, but the intellect."
Reply to Objection 2. The condition of past may be referred to two things--namely, to the object which is known, and to the act of knowledge. These two are found together in the sensitive part, which apprehends something from the fact of its being immuted by a present sensible: wherefore at the same time an animal remembers to have sensed before in the past, and to have sensed some past sensible thing. But as concerns the intellectual part, the past is accidental, and is not in itself a part of the object of the intellect. For the intellect understands man, as man: and to man, as man, it is accidental that he exist in the present, past, or future. But on the part of the act, the condition of past, even as such, may be understood to be in the intellect, as well as in the senses. Because our soul's act of understanding is an individual act, existing in this or that time, inasmuch as a man is said to understand now, or yesterday, or tomorrow. And this is not incompatible with the intellectual nature: for such an act of understanding, though something individual, is yet an immaterial act, as we have said above of the intellect (76, 1); and therefore, as the intellect understands itself, though it be itself an individual intellect, so also it understands its act of understanding, which is an individual act, in the past, present, or future. In this way, then, the notion of memory, in as far as it regards past events, is preserved in the intellect, forasmuch as it understands that it previously understood: but not in the sense that it understands the past as something "here" and "now."
Reply to Objection 3. The intelligible species is sometimes in the intellect only in potentiality, and then the intellect is said to be in potentiality. Sometimes the intelligible species is in the intellect as regards the ultimate completion of the act, and then it understands in act. And sometimes the intelligible species is in a middle state, between potentiality and act: and then we have habitual knowledge. In this way the intellect retains the species, even when it does not understand in act.
Source: Summa Theologica (New Advent)