The nature of cognition
For Aquinas, following Aristotle, cognition is an activity which knowing subject perform, not something that happens to them. Thus, contrary to the kinds of actions where one thing acts on another, (e.g. fire heating water) cognition is an action that an animal performs but which remains in the animal. It is, therefore, called an immanent action. As when water becomes hot from a fire, there is a change in us when we know, and it is a sort of union, as the fire and water approach the same temperature. As an immanent activity, though, we do not become merely like something else, as water becomes hot like the fire. Rather, in cognition we become the thing, in such a way that we do not cease being what we are. This way of becoming another is called the intentional order. In knowledge we become the intentionally the object known, and thus acquire a new perfection for ourselves, the same perfection of the things we know. And since, for Aquinas “form” is the principle of perfection, knowledge consists in acquiring or receiving the forms of the things we know and thereby becoming one with them.
The perfection belonging to one thing is found in another. This is the perfection of a knower insofar as he knows; for something is known by a knower by reason of the fact that the thing known is, in some fashion, in the possession of the knower. Hence it is said in The Soul that the soul is “in some manner, all things,” since its nature is such that it can know all things. In this way, it is possible for the perfection of the entire universe to exist in one thing.De veritate 2, 2
The union involved in knowledge.
Knowledge presupposes some kind of union, because in order to become the thing which is known we must possess it, we must be identical with the object we know. But this possession of the object is not a physical possession of it. It is a possession of the form of the object, of that principle which makes the object to be what it is. This is what Aristotle means when he says that the soul in a way becomes all things. Entitatively the knower and object known remain what they are. But intentionally (cognitively) the knower becomes the object of his knowledge as he possesses the form of the object, That is why Aquinas says with reference to intellectual knowledge:
Intelligent beings are distinguished from non-intelligent beings in that the latter possess only their own form; whereas the intelligent being is naturally adapted to have also the form of some other thing; for the idea of the thing known is in the knower.Summa Theologiae Ia, 14, 1
In the union of physical objects, the objects which are united retain their identity. When the union presupposes a substantial change, as in the process of nutrition, the food loses its identity to become the living organism.
In cognition, the actual knowledge is identical with its object. The knower, without losing its physical being, is identical with the object known because it possesses in a unique way the form of the object known. Both subject and object retain their identity, but the subject acquires an additional perfection: the form of the object known. The knower and the object known may be physically distinct while they are cognitively identical.
Follow this link further discussion of the Identity of the Knower and Known.
The foundation of knowledge: immateriality.
The knowing being is not limited to its own being, but it is capable of being other than itself, while the non-knowing being is limited to its own being. The knower has, in addition to his own form, the form of the thing known. What makes knowing beings capable of having the forms of other beings?
Non-intelligent beings possess their own form; whereas the intelligent being is naturally adapted to have also the form of some other thing; for the idea of the thing know is in the knower. Hence it is manifest that the nature of the non-intelligent beings is more contracted and limited; whereas the nature of intelligent beings has a greater amplitude and extension; the soul is in a sense all things. Now the contraction of the form comes from matter… Forms, according as they are more immaterial, approach more nearly to a kind of infinity. Therefore it is clear that the immateriality of a thing is the reason why it is cognitive; and the mode of knowledge is according to the mode of immateriality.ST I, 14, 1
Aquinas explains the sentence, “the contraction of the form comes from the matter,” in this way:
Form is made finite by matter, inasmuch as form, considered in itself, is common to many; but when received in matter, the form is determined to this one particular thing… Form is not made perfect by matter, but is rather contracted by matter.ST I, 7, 1
In other words, when a form informs physical matter, that form is restricted exclusively to that matter: material things known must exist in the knower, not materially, but immaterially.
The reason for this is that the act of knowledge extends to things outside the knower: for we know even things that are external to us. Now by matter, the form of a thing is determined to some one thing. Wherefore it is clear that knowledge is in inverse ratio of materiality. And consequently things ‘such as plants’ that are not receptive of forms save materially, have no power of knowledge whatever… But the more immaterially a thing receives the form of the thing known, the more perfect is its knowledge.ST I, 84, 2. cf: In de anima II, lect.24, n.551, ff; De ver., 2, 2.
Anything which receives in a purely material way, as a passive potency, is limited to one form at a time; matter is the principle of individuation.
In the physical order (the existential order), the forms are restricted. Only one form can exist in “this” or “that” physical individual matter. In the intentional order, the cognitive power receives the forms abstracted from the physical individual matter, and therefore free from the element (matter) that reduces them to “this” or “that” individual.
The immateriality which is the foundation of knowledge should not be confused with that immateriality which corresponds to spiritual substances (e.g., the angels of Thomas’ philosophy or the “incorporeal substances” of Aristotle’s philosophy). Such substances are entirely without matter. The immateriality which is the root of knowledge implies only a certain independence from matter which permits the forms to become the objects of the cognitive power.
The impressed and expressed species in the process of cognition.
Cognition is an act by which the knower actually becomes the object known. It is impossible to know an object unless we have in a cognitive way the form of the object we know. For this reason the ancients believed that, since we are capable of knowing all things, our soul is in some way composed of “all things”:
Some philosophers–and this, as we have seen, was the view of Empedocles–thought that intellect was a composition of all the principles of things, and this explained its universal knowledge.In de anima III, lect. 7, n.677.
Since the soul is not “all things”, we have to acquire a likeness of the real form, or knowledge would be impossible. This likeness of the real form is called the “species.” (Remember, however, that the species is not just a “little picture” of the object. As a word (“tree”) may represent an object (a tree) and as a calling card may represent a businessman, so the species may represent an object without looking like that object.)
There are two kinds of species, called the “impressed species,” and the “expressed species.” The general reason for the need of “impressed species” is this: a finite being is first potentially that which it comes to be actually. The knower becomes the object potentially by means of the impressed species which specifies and determines the cognitive faculty to be the object of which the species is the similitude.
The created intellect cannot understand any substance unless it becomes actual by means of some species, which is the likeness of the thing understood, informing it….Summa Contra Gentiles III, c.51. nr.3.
Our passive intellect has the same relation to intelligible objects as primary matter has to natural things; for it is in potentiality to intelligible objects, just as primary matter is to natural things. Hence our passive intellect can be exercised concerning intelligible objects only so far as it perfected by the intelligible species of something.”ST I, 14, 2, ad.3. cf: R. Lambert, “A Textual Study of Aquinas’ Comparison of the Intellect to Prime Matter,” New Scholasticism 55(1982) 80-99.
Thus the impressed species actuates the cognitive potency and so renders it fully capable of eliciting its act. In doing so, it also specifies that particular act so that the act includes one object and no other.
The mere reception of an impressed species is not yet cognition. For this reception is purely passive, while cognition is an active immanent action. The impressed species is not that which (quod) is known, but that by means of which (quo) the object is known.(See De pot. 8, 1, co.)
It is clear that the species by which the intellect is actualized are not in themselves the intellect’s object, for they are not that which, but that by which it understands.In de anima, III, lect. 4, n.78.
The species is not that which is understood… In the act of understanding, the species function as the thing by which one understands, and not as that which is understood, even as the species of color in the eye is not that which is seen, but that by which we see. And that which is understood is the very intelligible essence of things existing outside the soul, just as things outside the soul are seen by corporeal light.SCG II, c.75, nr.7. cf: ST I, 85, 2.
The intellect forms a new species called the expressed species, the natural word.
The intelligible species is the likeness of the thing understood, which likeness informs the intellect for the purpose of understanding. For the intellect cannot understand except insofar as it is actuated by this likeness, just as nothing else can act as being in potentiality but only as actuated by a form. Accordingly, this likeness is as the principle in the act of understanding, and not as the term of understanding. Consequently that which is the first and direct object in the act of understanding is something that the intellect conceives within itself about the thing understood, whether it be a definition or proposition according to the two operations of the intellect mentioned in De anima III. Now this concept of the intellect is called the interior word and is signified by means of speech: for the spoken word does not signify merely the thing understood, but the concept of the intellect through which it signifies the thing.De pot, 9, 5, co.
An external thing understood by us does not exist in our intellect according to its own nature; rather, it is necessary that its species be in our intellect, and through this species the intellect comes to be in act. Once in act through this species as through its own form, the intellect knows the thing itself… Understanding remains in the one understanding, but it is related to the thing understood because the above mentioned species, which is a principle of intellectual operation as a form, is the likeness of the thing understood.
The intellect, having been informed by the species of the thing, by an act of understanding forms within itself a certain intention of the thing understood, that is to say, its notion, which the definition signifies… This is a necessary point, because the intellect understands a present and an absent thing indifferently. In this the imagination agrees with the intellect. But the intellect has this characteristic in addition, namely, that it understands a thing as separated from material conditions, without which a thing does not exist in reality. But this could not take place unless the intellect formed the above mentioned intention for itself.
Now, since this understood intention is, as it were, a terminus of intelligible operation, it is distinct from the intelligible species that actualizes the intellect and that we must consider the principle of intellectual operation, though both are a likeness of the thing understood. For, by the fact that the intelligible species, which is the form of the intellect and the principle of understanding, is the likeness of the external thing, it follows that the intellect forms an intention like that thing, since such as a thing is, such are its works. And because the understood intention is like some thing, it follows that the intellect, by forming such an intention, knows that thing.SCG I, c.53, nos. 2-4. cf: SCG I, c.59, no. 2; SCG IV, c.11, no. 5.
Another reason for the necessity of the expressed species lies in the distinction between the being and its operation. (See ST I, q.56, B.A.C. edition, Introduction, p.252, and ST I, 27, 1, and SCG IV, c.11.)