Objection 1. It would seem that our intellect does not understand corporeal and material things by abstraction from the phantasms. For the intellect is false if it understands an object otherwise than as it really is. Now the forms of material things do not exist as abstracted from the particular things represented by the phantasms. Therefore, if we understand material things by abstraction of the species from the phantasm, there will be error in the intellect.
Objection 2. Further, material things are those natural things which include matter in their definition. But nothing can be understood apart from that which enters into its definition. Therefore material things cannot be understood apart from matter. Now matter is the principle of individualization. Therefore material things cannot be understood by abstraction of the universal from the particular, which is the process whereby the intelligible species is abstracted from the phantasm.
Objection 3. Further, the Philosopher says (De Anima iii, 7) that the phantasm is to the intellectual soul what color is to the sight. But seeing is not caused by abstraction of species from color, but by color impressing itself on the sight. Therefore neither does the act of understanding take place by abstraction of something from the phantasm, but by the phantasm impressing itself on the intellect.
Objection 4. Further, the Philosopher says (De Anima iii, 5) there are two things in the intellectual soul--the passive intellect and the active intellect. But it does not belong to the passive intellect to abstract the intelligible species from the phantasm, but to receive them when abstracted. Neither does it seem to be the function of the active intellect, which is related to the phantasm, as light is to color; since light does not abstract anything from color, but rather streams on to it. Therefore in no way do we understand by abstraction from phantasms.
Objection 5. Further, the Philosopher (De Anima iii, 7) says that "the intellect understands the species in the phantasm"; and not, therefore, by abstraction.
On the contrary, The Philosopher says (De Anima iii, 4) that "things are intelligible in proportion as they are separate from matter." Therefore material things must needs be understood according as they are abstracted from matter and from material images, namely, phantasms.
I answer that, As stated above (Question 84, Article 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 (Question 76, Article 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.
Reply to Objection 2. Some have thought that the species of a natural thing is a form only, and that matter is not part of the species. If that were so, matter would not enter into the definition of natural things. Therefore it must be said otherwise, that matter is twofold, common, and "signate" or individual; common, such as flesh and bone; and individual, as this flesh and these bones. The intellect therefore abstracts the species of a natural thing from the individual sensible matter, but not from the common sensible matter; for example, it abstracts the species of man from "this flesh and these bones," which do not belong to the species as such, but to the individual (Metaph. vii, Did. vi, 10), and need not be considered in the species: whereas the species of man cannot be abstracted by the intellect form "flesh and bones."
Mathematical species, however, can be abstracted by the intellect from sensible matter, not only from individual, but also from common matter; not from common intelligible matter, but only from individual matter. For sensible matter is corporeal matter as subject to sensible qualities, such as being cold or hot, hard or soft, and the like: while intelligible matter is substance as subject to quantity. Now it is manifest that quantity is in substance before other sensible qualities are. Hence quantities, such as number, dimension, and figures, which are the terminations of quantity, can be considered apart from sensible qualities; and this is to abstract them from sensible matter; but they cannot be considered without understanding the substance which is subject to the quantity; for that would be to abstract them from common intelligible matter. Yet they can be considered apart from this or that substance; for that is to abstract them from individual intelligible matter. But some things can be abstracted even from common intelligible matter, such as "being," "unity," "power," "act," and the like; all these can exist without matter, as is plain regarding immaterial things. Because Plato failed to consider the twofold kind of abstraction, as above explained (ad 1), he held that all those things which we have stated to be abstracted by the intellect, are abstract in reality.
Reply to Objection 3. Colors, as being in individual corporeal matter, have the same mode of existence as the power of sight: therefore they can impress their own image on the eye. But phantasms, since they are images of individuals, and exist in corporeal organs, have not the same mode of existence as the human intellect, and therefore have not the power of themselves to make an impression on the passive intellect. This is done by the power of the active intellect which by turning towards the phantasm produces in the passive intellect a certain likeness which represents, as to its specific conditions only, the thing reflected in the phantasm. It is thus that the intelligible species is said to be abstracted from the phantasm; not that the identical form which previously was in the phantasm is subsequently in the passive intellect, as a body transferred from one place to another.
Reply to Objection 4. Not only does the active intellect throw light on the phantasm: it does more; by its own power it abstracts the intelligible species from the phantasm. It throws light on the phantasm, because, just as the sensitive part acquires a greater power by its conjunction with the intellectual part, so by the power of the active intellect the phantasms are made more fit for the abstraction therefrom of intelligible intentions. Furthermore, the active intellect abstracts the intelligible species from the phantasm, forasmuch as by the power of the active intellect we are able to disregard the conditions of individuality, and to take into our consideration the specific nature, the image of which informs the passive intellect.
Reply to Objection 5. Our intellect both abstracts the intelligible species from the phantasms, inasmuch as it considers the natures of things in universal, and, nevertheless, understands these natures in the phantasms since it cannot understand even the things of which it abstracts the species, without turning to the phantasms, as we have said above (Question 84, Article 7).
Source: Summa Theologica (New Advent)