Since there are in man two cognitive powers, namely sense and intellect, it is manifest that these presuppose a diversity on the side of their objects. Now it must be known that in human cognition the foundation and origin is sense, whence it is nearer to the external thing upon which the whole action of the sensitive powers is founded, according to the Philosopher in the book Perihermeneias. Hence, on account of the manifest passion of the senses arising from sensible things as from something near, he said that the passions of the intellect are likenesses of things, but of the passions of the senses he made there no mention.
This, however, was done chiefly because the intellective passions are causes of vocal sounds more immediately than the passions of the senses, of which he then intended to treat. Now it is certain that the accidents themselves, whether proper or common, are sensibles, that is, objects of the senses, as is held in the second book of On the Soul. Hence, since one power is not borne upon the proper object of another power except by accident—as the eye sees the sweet insofar as it is white or black—it is impossible for the intellect to be borne upon those accidents except by accident, namely insofar as it acts upon the very phantasms which are on the side of those sensible accidents; and this is by way of a certain reflection. Now reflection is twofold:
one from the origin of the power, the other from the origin of the object. For first, any power is borne upon its own proper object, and that has the role of determining the act of the power itself; hence the act of the power does not pass beyond the further form of its more principal object; for otherwise that would not be its proper object; for the proper object is that by whose informing the power exercises its act, as color is related to sight, and sound to hearing. But the quiddity of a sensible thing is the proper object of the intellect, as is said in the third book of On the Soul: hence, wheresoever the soul turns itself, this object is the ground of acting for it.
It can therefore either return into itself through its own act and power, or return through the object into the very origin of the object, namely through the phantasms into the species of sensible things. Therefore these are its two modes of reflection, of which one knows itself and the things that are on its own side; while the other knows the things that are on the side of its object. In each mode, however, its cognition is terminated at something singular. For when its gaze is fixed upon its very object, it apprehends the account of the universal, which alone among these lower things is determined by the intellect as its proper object, since all singulars among us are material: for matter impedes the intellect, but the singular does not:
for matter is knowable only by analogy to form, as is said in the first book of the Physics. But if there were among us a singular that was insensible and immaterial—which, however, is impossible—it would be known through itself without any abstraction, because singularity does not impede the intellect, but materiality does. When therefore the intellect tends toward the origin of its object, namely toward sensible things, it is reflected back upon the objects of the other cognitive powers. But when it returns to itself, it does not tend toward the object of any other power:
because the potentiality of the soul, by reason of which it cannot itself be the complete object of its own proper intellect, unless it is brought to act through the species of a principal object—as the Philosopher teaches in the third book of On the Soul, since the soul understands itself just as it understands other things. But it understands other things through a species. Therefore it is not materiality that makes it be as an object of another power; yet it does make a certain mode of singularity—not such as belongs to some thing complete in species, but because the soul's existence is terminated by the receptivity of its nature, whose potentiality is greater than the potentiality of the angels, because it is more distant from God in the grade of nature than an angel. For God is pure act:
and the less anything approaches Him, the more potentiality and the less actuality it has: for not only matter, but also any passive potentiality whatsoever, has less of the account of the object of the intellect: because, as the Philosopher says in the ninth book of the Metaphysics, each thing is understood according as it is a being in act. Hence the soul has a twofold mode of cognition with respect to objects. One within itself, as it is taken in its separate state: for then it is knowable according to the mode of its existence, because it is an act perfecting its own potentiality. For in the soul there are not two acts, namely its essence which is its potentiality, and its existence; but its existence is its actuality, by reason of which the very essence of the soul is the act of the body:
for every form participates in act, which is first through essence; but this comes about analogically. Now it is impossible for any one thing to participate at once in two acts of the same order, just as it is impossible for the same matter to participate at once in two substantial forms. But the soul has another mode of cognition from itself, insofar as it is born to know through the senses: hence it is not knowable, in the state in which something is received through sense, except through the species of a sensible thing; and therefore all its cognition of itself is properly reflexive. Now therefore it remains to show where the cognition of sense and of intellect are separated from one another. For it is manifest that the intellect begins where sense leaves off.
Now the external senses have the sensible accidents themselves, namely the common and the proper, for their own per se objects. But the quiddity of a particular thing in the particular is not regarded as a per se object by those external senses, since that quiddity is a substance and not an accident; nor does it belong to the intellect as its per se object, on account of its materiality. Therefore the quiddity of a material thing in its very particularity is the object of the particular reason, whose function is to compare among particular intentions: in place of which, in brute animals, there is the natural estimative power. This power, by its conjunction with the intellect—where reason itself is, which compares among universals—participates in a comparative power:
but because it is a part of the sensitive power, it does not abstract altogether from matter. Hence its proper object remains a particular material quiddity. This, however, is not that this power apprehends matter in itself, since matter cannot be known except by analogy to form; but because the comparison concerning matter in relation to form, through matter itself individuated, belongs to this power, just as to consider matter in common in relation to the form of a species belongs to the higher reason. From these things it is easy to see how matter is the principle of individuation: for this is what we intend to show. It must therefore be known that the individual, among us, consists in two things.
For the individual in sensible things is that very last thing in the genus of substance, which is predicated of no other: nay rather, it itself is a first substance, according to the Philosopher in the Categories, and the first foundation of all others. For the nature of a material form, since it cannot itself be a complete 'this something' in a species—whose existence alone is incommunicable—is communicable as far as concerns its own account; but it is incommunicable only by reason of the supposit, which is something complete in a species, which does not belong to every form, as has been said. Therefore, as far as concerns its own account, it is communicable, as has been said. Now its communication is, as has been said, through the fact that it is received in others.
Therefore, as far as concerns its own nature, it is communicable and can be received in many, and it is received according to one account, since there is one account of the species in all its individuals. But because it does not itself have existence, as has been said—since existence belongs only to the supposit, and the supposit is incommunicable, as has been said—therefore the material form itself is diversified according to many incommunicable existences, while remaining one according to an account communicated to many: but its reception is in matter, because it itself is material. From this it is clear that, by its own nature, unity of account in its communication is left to it, and that it is rendered incommunicable through its reception in matter.
For from the fact that it is received in matter, it is made an individual, which is incommunicable, and the first foundation in the genus of substance, as a complete subject of the others predicable of it. But in the path of generation what is incomplete is always prior to what is complete, although in the path of perfection the whole is, on the contrary. That therefore which is the first subject of all in the path of generation, and incomplete, which is predicated of nothing of that genus—namely matter—must necessarily be the first principle of incommunicable existence, which is proper to the individual.
There is another thing in which the account of the individual is preserved among us, namely its determination to certain particles of time and place, because it is proper to it to be 'here' and 'now,' and this determination is due to it by reason of determinate quantity. And therefore matter under determinate quantity is the principle of individuation: for matter alone is the first principle of individuation, as regards that in which the account of what is first in the genus of substance is preserved—which, however, is impossible to be found without body and quantity. And therefore determinate quantity is called the principle of individuation, not because it in any way causes its own subject, which is a first substance, but because it accompanies it inseparably, and determines it to a 'here' and 'now.'
That therefore which falls under the particular account is a 'this something' through the nature of matter; but that which falls under the external sense is so through quantity. But against this it can be objected that matter, by its own nature, is common, just as form is, since one matter can be under several forms. Hence this commonness of it can impede it from being the first principle of individuation. But it must be known that it is impossible for a form to be united to matter without its being particular and without determinate quantity following upon it, by which mode the matter is no longer further communicable to another form, because this quantity cannot be found, with the same determination, together with another form.
And therefore matter is not communicable according to the same determination, as form is, as has been said. For the account of matter under one form is other than its account under another form, because its account is made certain through the determination of quantity, which in a different proportion and dimension is required for different forms. The essence of matter, however, is not diversified under diverse forms as its account is: for the account of matter is not one and common as its essence is: for the account of a form in diverse matters is one, although according to existence it is diverse.
And therefore commonality, taken only according to the essence which belongs to matter itself and not according to the same intelligible character, does not prevent matter from being the first principle of individuation. But it must be noted that accidents are not individuated through prime matter, but are individuated through their proper subject, which is an actual being, just as substantial forms are individuated through prime matter, which is their subject. And these remarks concerning the principle of individuation are sufficient for the present.