I answer that, Some have held that the intelligible species of our intellect are derived from certain separate forms or substances. And this in two ways. For Plato, as we have said (1), held that the forms of sensible things subsist by themselves without matter; for instance, the form of a man which he called "per se" man, and the form or idea of a horse which is called "per se" horse, and so forth. He said therefore that these forms are participated both by our soul and by corporeal matter; by our soul, to the effect of knowledge thereof, and by corporeal matter to the effect of existence: so that, just as corporeal matter by participating the idea of a stone, becomes an individuating stone, so our intellect, by participating the idea of a stone, is made to understand a stone. Now participation of an idea takes place by some image of the idea in the participator, just as a model is participated by a copy. So just as he held that the sensible forms, which are in corporeal matter, are derived from the ideas as certain images thereof: so he held that the intelligible species of our intellect are images of the ideas, derived therefrom. And for this reason, as we have said above (1), he referred sciences and definitions to those ideas.

But in this opinion no sufficient reason can be assigned for the soul being united to the body. For it cannot be said that the intellectual soul is united to the body for the sake of the body: for neither is form for the sake of matter, nor is the mover for the sake of the moved, but rather the reverse. Especially does the body seem necessary to the intellectual soul, for the latter's proper operation which is to understand: since as to its being the soul does not depend on the body. But if the soul by its very nature had an inborn aptitude for receiving intelligible species through the influence of only certain separate principles, and were not to receive them from the senses, it would not need the body in order to understand: wherefore to no purpose would it be united to the body.

But if it be said that our soul needs the senses in order to understand, through being in some way awakened by them to the consideration of those things, the intelligible species of which it receives from the separate principles: even this seems an insufficient explanation. For this awakening does not seem necessary to the soul, except in as far as it is overcome by sluggishness, as the Platonists expressed it, and by forgetfulness, through its union with the body: and thus the senses would be of no use to the intellectual soul except for the purpose of removing the obstacle which the soul encounters through its union with the body. Consequently the reason of the union of the soul with the body still remains to be sought.

Objection 3. There are no phantasms of incorporeal things: for the imagination does not transcend time and space. If, therefore, our intellect cannot understand anything actually without turning to the phantasms, it follows that it cannot understand anything incorporeal. Which is clearly false: for we understand truth, and God, and the angels.

I answer that, In the present state of life in which the soul is united to a passible body, it is impossible for our intellect to understand anything actually, except by turning to the phantasms.

First of all because the intellect, being a power that does not make use of a corporeal organ, would in no way be hindered in its act through the lesion of a corporeal organ, if for its act there were not required the act of some power that does make use of a corporeal organ. Now sense, imagination and the other powers belonging to the sensitive part, make use of a corporeal organ. Wherefore it is clear that for the intellect to understand actually, not only when it acquires fresh knowledge, but also when it applies knowledge already acquired, there is need for the act of the imagination and of the other powers. For when the act of the imagination is hindered by a lesion of the corporeal organ, for instance in a case of frenzy; or when the act of the memory is hindered, as in the case of lethargy, we see that a man is hindered from actually understanding things of which he had a previous knowledge.

Secondly, anyone can experience this of himself, that when he tries to understand something, he forms certain phantasms to serve him by way of examples, in which as it were he examines what he is desirous of understanding. For this reason it is that when we wish to help someone to understand something, we lay examples before him, from which he forms phantasms for the purpose of understanding.

Now the reason of this is that the power of knowledge is proportioned to the thing known. Wherefore the proper object of the angelic intellect, which is entirely separate from a body, is an intelligible substance separate from a body. Whereas the proper object of the human intellect, which is united to a body, is a quiddity or nature existing in corporeal matter; and through such natures of visible things it rises to a certain knowledge of things invisible. Now it belongs to such a nature to exist in an individual, and this cannot be apart from corporeal matter: for instance, it belongs to the nature of a stone to be in an individual stone, and to the nature of a horse to be in an individual horse, and so forth. Wherefore the nature of a stone or any material thing cannot be known completely and truly, except in as much as it is known as existing in the individual. Now we apprehend the individual through the senses and the imagination. And, therefore, for the intellect to understand actually its proper object, it must of necessity turn to the phantasms in order to perceive the universal nature existing in the individual. But if the proper object of our intellect were a separate form; or if, as the Platonists say, the natures of sensible things subsisted apart from the individual; there would be no need for the intellect to turn to the phantasms whenever it understands.

Reply to Objection 1. The species preserved in the passive intellect exist there habitually when it does not understand them actually, as we have said above (79, 6). Wherefore for us to understand actually, the fact that the species are preserved does not suffice; we need further to make use of them in a manner befitting the things of which they are the species, which things are natures existing in individuals.

I answer that, As stated above (84, 7), the object of knowledge is proportionate to the power of knowledge. Now there are three grades of the cognitive powers. For one cognitive power, namely, the sense, is the act of a corporeal organ. And therefore the object of every sensitive power is a form as existing in corporeal matter. And since such matter is the principle of individuality, therefore every power of the sensitive part can only have knowledge of the individual. There is another grade of cognitive power which is neither the act of a corporeal organ, nor in any way connected with corporeal matter; such is the angelic intellect, the object of whose cognitive power is therefore a form existing apart from matter: for though angels know material things, yet they do not know them save in something immaterial, namely, either in themselves or in God. But the human intellect holds a middle place: for it is not the act of an organ; yet it is a power of the soul which is the form the body, as is clear from what we have said above (76, 1). And therefore it is proper to it to know a form existing individually in corporeal matter, but not as existing in this individual matter. But to know what is in individual matter, not as existing in such matter, is to abstract the form from individual matter which is represented by the phantasms. Therefore we must needs say that our intellect understands material things by abstracting from the phantasms; and through material things thus considered we acquire some knowledge of immaterial things, just as, on the contrary, angels know material things through the immaterial.

But Plato, considering only the immateriality of the human intellect, and not its being in a way united to the body, held that the objects of the intellect are separate ideas; and that we understand not by abstraction, but by participating things abstract, as stated above (84 , 1).

Reply to Objection 1. Abstraction may occur in two ways:

First, by way of composition and division; thus we may understand that one thing does not exist in some other, or that it is separate therefrom.

Secondly, by way of simple and absolute consideration; thus we understand one thing without considering the other. Thus for the intellect to abstract one from another things which are not really abstract from one another, does, in the first mode of abstraction, imply falsehood. But, in the second mode of abstraction, for the intellect to abstract things which are not really abstract from one another, does not involve falsehood, as clearly appears in the case of the senses. For if we understood or said that color is not in a colored body, or that it is separate from it, there would be error in this opinion or assertion. But if we consider color and its properties, without reference to the apple which is colored; or if we express in word what we thus understand, there is no error in such an opinion or assertion, because an apple is not essential to color, and therefore color can be understood independently of the apple. Likewise, the things which belong to the species of a material thing, such as a stone, or a man, or a horse, can be thought of apart from the individualizing principles which do not belong to the notion of the species. This is what we mean by abstracting the universal from the particular, or the intelligible species from the phantasm; that is, by considering the nature of the species apart from its individual qualities represented by the phantasms. If, therefore, the intellect is said to be false when it understands a thing otherwise than as it is, that is so, if the word "otherwise" refers to the thing understood; for the intellect is false when it understands a thing otherwise than as it is; and so the intellect would be false if it abstracted the species of a stone from its matter in such a way as to regard the species as not existing in matter, as Plato held. But it is not so, if the word "otherwise" be taken as referring to the one who understands. For it is quite true that the mode of understanding, in one who understands, is not the same as the mode of a thing in existing: since the thing understood is immaterially in the one who understands, according to the mode of the intellect, and not materially, according to the mode of a material thing.