Objection 1. It would seem that the interior senses are not suitably distinguished. For the common is not divided against the proper. Therefore the common sense should not be numbered among the interior sensitive powers, in addition to the proper exterior senses.
Objection 2. Further, there is no need to assign an interior power of apprehension when the proper and exterior sense suffices. But the proper and exterior senses suffice for us to judge of sensible things; for each sense judges of its proper object. In like manner they seem to suffice for the perception of their own actions; for since the action of the sense is, in a way, between the power and its object, it seems that sight must be much more able to perceive its own vision, as being nearer to it, than the color; and in like manner with the other senses. Therefore for this there is no need to assign an interior power, called the common sense.
Objection 3. Further, according to the Philosopher (De Memor. et Remin. i), the imagination and the memory are passions of the "first sensitive." But passion is not divided against its subject. Therefore memory and imagination should not be assigned as powers distinct from the senses.
Objection 4. Further, the intellect depends on the senses less than any power of the sensitive part. But the intellect knows nothing but what it receives from the senses; whence we read (Poster. i, 8), that "those who lack one sense lack one kind of knowledge." Therefore much less should we assign to the sensitive part a power, which they call the "estimative" power, for the perception of intentions which the sense does not perceive.
Objection 5. Further, the action of the cogitative power, which consists in comparing, adding and dividing, and the action of the reminiscence, which consists in the use of a kind of syllogism for the sake of inquiry, is not less distant from the actions of the estimative and memorative powers, than the action of the estimative is from the action of the imagination. Therefore either we must add the cognitive and reminiscitive to the estimative and memorative powers, or the estimative and memorative powers should not be made distinct from the imagination.
Objection 6. Further, Augustine (Gen. ad lit. xii, 6,7,24) describes three kinds of vision; namely, corporeal, which is the action of the sense; spiritual, which is an action of the imagination or phantasy; and intellectual, which is an action of the intellect. Therefore there is no interior power between the sense and intellect, besides the imagination.
On the contrary, Avicenna (De Anima iv, 1) assigns five interior sensitive powers; namely, "common sense, phantasy, imagination, and the estimative and memorative powers."
I answer that, As nature does not fail in necessary things, there must needs be as many actions of the sensitive soul as may suffice for the life of a perfect animal. If any of these actions cannot be reduced to the same one principle, they must be assigned to diverse powers; since a power of the soul is nothing else than the proximate principle of the soul's operation.
Now we must observe that for the life of a perfect animal, the animal should apprehend a thing not only at the actual time of sensation, but also when it is absent. Otherwise, since animal motion and action follow apprehension, an animal would not be moved to seek something absent: the contrary of which we may observe specially in perfect animals, which are moved by progression, for they are moved towards something apprehended and absent. Therefore an animal through the sensitive soul must not only receive the species of sensible things, when it is actually affected by them, but it must also retain and preserve them. Now to receive and retain are, in corporeal things, reduced to diverse principles; for moist things are apt to receive, but retain with difficulty, while it is the reverse with dry things. Wherefore, since the sensitive power is the act of a corporeal organ, it follows that the power which receives the species of sensible things must be distinct from the power which preserves them.
Again we must observe that if an animal were moved by pleasing and disagreeable things only as affecting the sense, there would be no need to suppose that an animal has a power besides the apprehension of those forms which the senses perceive, and in which the animal takes pleasure, or from which it shrinks with horror. But the animal needs to seek or to avoid certain things, not only because they are pleasing or otherwise to the senses, but also on account of other advantages and uses, or disadvantages: just as the sheep runs away when it sees a wolf, not on account of its color or shape, but as a natural enemy: and again a bird gathers together straws, not because they are pleasant to the sense, but because they are useful for building its nest. Animals, therefore, need to perceive such intentions, which the exterior sense does not perceive. And some distinct principle is necessary for this; since the perception of sensible forms comes by an immutation caused by the sensible, which is not the case with the perception of those intentions.
Thus, therefore, for the reception of sensible forms, the "proper sense" and the "common sense" are appointed, and of their distinction we shall speak farther on (ad 1,2). But for the retention and preservation of these forms, the "phantasy" or "imagination" is appointed; which are the same, for phantasy or imagination is as it were a storehouse of forms received through the senses. Furthermore, for the apprehension of intentions which are not received through the senses, the "estimative" power is appointed: and for the preservation thereof, the "memorative" power, which is a storehouse of such-like intentions. A sign of which we have in the fact that the principle of memory in animals is found in some such intention, for instance, that something is harmful or otherwise. And the very formality of the past, which memory observes, is to be reckoned among these intentions.
Now, we must observe that as to sensible forms there is no difference between man and other animals; for they are similarly immuted by the extrinsic sensible. But there is a difference as to the above intentions: for other animals perceive these intentions only by some natural instinct, while man perceives them by means of coalition of ideas. Therefore the power by which in other animals is called the natural estimative, in man is called the "cogitative," which by some sort of collation discovers these intentions. Wherefore it is also called the "particular reason," to which medical men assign a certain particular organ, namely, the middle part of the head: for it compares individual intentions, just as the intellectual reason compares universal intentions. As to the memorative power, man has not only memory, as other animals have in the sudden recollection of the past; but also "reminiscence" by syllogistically, as it were, seeking for a recollection of the past by the application of individual intentions. Avicenna, however, assigns between the estimative and the imaginative, a fifth power, which combines and divides imaginary forms: as when from the imaginary form of gold, and imaginary form of a mountain, we compose the one form of a golden mountain, which we have never seen. But this operation is not to be found in animals other than man, in whom the imaginative power suffices thereto. To man also does Averroes attribute this action in his book De sensu et sensibilibus (viii). So there is no need to assign more than four interior powers of the sensitive part--namely, the common sense, the imagination, and the estimative and memorative powers.
Reply to Objection 1. The interior sense is called "common" not by predication, as if it were a genus; but as the common root and principle of the exterior senses.
Reply to Objection 2. The proper sense judges of the proper sensible by discerning it from other things which come under the same sense; for instance, by discerning white from black or green. But neither sight nor taste can discern white from sweet: because what discerns between two things must know both. Wherefore the discerning judgment must be assigned to the common sense; to which, as to a common term, all apprehensions of the senses must be referred: and by which, again, all the intentions of the senses are perceived; as when someone sees that he sees. For this cannot be done by the proper sense, which only knows the form of the sensible by which it is immuted, in which immutation the action of sight is completed, and from immutation follows another in the common sense which perceives the act of vision.
Reply to Objection 3. As one power arises from the soul by means of another, as we have seen above (Question 77, Article 7), so also the soul is the subject of one power through another. In this way the imagination and the memory are called passions of the "first sensitive."
Reply to Objection 4. Although the operation of the intellect has its origin in the senses: yet, in the thing apprehended through the senses, the intellect knows many things which the senses cannot perceive. In like manner does the estimative power, though in a less perfect manner.
Reply to Objection 5. The cogitative and memorative powers in man owe their excellence not to that which is proper to the sensitive part; but to a certain affinity and proximity to the universal reason, which, so to speak, overflows into them. Therefore they are not distinct powers, but the same, yet more perfect than in other animals.
Reply to Objection 6. Augustine calls that vision spiritual which is effected by the images of bodies in the absence of bodies. Whence it is clear that it is common to all interior apprehensions.
Source: Summa Theologica (New Advent)