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Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We have now to consider how the intellectual soul knows itself and all within itself. Under this head there are four points of inquiry:
(1) Whether the soul knows itself by its own essence?
(2) Whether it knows its own habits?
(3) How does the intellect know its own act?
(4) How does it know the act of the will?
Objection 1: It would seem that the intellectual soul knows itself by its own essence. For Augustine says (De Trin. ix, 3), that "the mind knows itself, because it is incorporeal."
Objection 2: Further, both angels and human souls belong to the genus of intellectual substance. But an angel understands itself by its own essence. Therefore likewise does the human soul.
Objection 3: Further, "in things void of matter, the intellect and that which is understood are the same" (De Anima iii, 4). But the human mind is void of matter, not being the act of a body as stated above (Q[76], A[1]). Therefore the intellect and its object are the same in the human mind; and therefore the human mind understands itself by its own essence.
On the contrary, It is said (De Anima iii, 4) that "the intellect understands itself in the same way as it understands other things." But it understands other things, not by their essence, but by their similitudes. Therefore it does not understand itself by its own essence.
I answer that, Everything is knowable so far as it is in act, and not, so far as it is in potentiality (Metaph. ix, Did. viii, 9): for a thing is a being, and is true, and therefore knowable, according as it is actual. This is quite clear as regards sensible things, for the eye does not see what is potentially, but what is actually colored. In like manner it is clear that the intellect, so far as it knows material things, does not know save what is in act: and hence it does not know primary matter except as proportionate to form, as is stated Phys. i, 7. Consequently immaterial substances are intelligible by their own essence according as each one is actual by its own essence.
Therefore it is that the Essence of God, the pure and perfect act, is simply and perfectly in itself intelligible; and hence God by His own Essence knows Himself, and all other things also. The angelic essence belongs, indeed, to the genus of intelligible things as "act," but not as a "pure act," nor as a "complete act," and hence the angel's act of intelligence is not completed by his essence. For although an angel understands himself by his own essence, still he cannot understand all other things by his own essence; for he knows things other than himself by their likenesses. Now the human intellect is only a potentiality in the genus of intelligible beings, just as primary matter is a potentiality as regards sensible beings; and hence it is called "possible" [*Possibilis---elsewhere in this translation rendered "passive"---Ed.]. Therefore in its essence the human mind is potentially understanding. Hence it has in itself the power to understand, but not to be understood, except as it is made actual. For even the Platonists asserted than an order of intelligible beings existed above the order of intellects, forasmuch as the intellect understands only by participation of the intelligible; for they said that the participator is below what it participates. If, therefore, the human intellect, as the Platonists held, became actual by participating separate intelligible forms, it would understand itself by such participation of incorporeal beings. But as in this life our intellect has material and sensible things for its proper natural object, as stated above (Q[84], A[7]), it understands itself according as it is made actual by the species abstracted from sensible things, through the light of the active intellect, which not only actuates the intelligible things themselves, but also, by their instrumentality, actuates the passive intellect. Therefore the intellect knows itself not by its essence, but by its act. This happens in two ways: In the first place, singularly, as when Socrates or Plato perceives that he has an intellectual soul because he perceives that he understands. In the second place, universally, as when we consider the nature of the human mind from knowledge of the intellectual act. It is true, however, that the judgment and force of this knowledge, whereby we know the nature of the soul, comes to us according to the derivation of our intellectual light from the Divine Truth which contains the types of all things as above stated (Q[84], A[5]). Hence Augustine says (De Trin. ix, 6): "We gaze on the inviolable truth whence we can as perfectly as possible define, not what each man's mind is, but what it ought to be in the light of the eternal types." There is, however, a difference between these two kinds of knowledge, and it consists in this, that the mere presence of the mind suffices for the first; the mind itself being the principle of action whereby it perceives itself, and hence it is said to know itself by its own presence. But as regards the second kind of knowledge, the mere presence of the mind does not suffice, and there is further required a careful and subtle inquiry. Hence many are ignorant of the soul's nature, and many have erred about it. So Augustine says (De Trin. x, 9), concerning such mental inquiry: "Let the mind strive not to see itself as if it were absent, but to discern itself as present"---i.e. to know how it differs from other things; which is to know its essence and nature.
Reply to Objection 1: The mind knows itself by means of itself, because at length it acquires knowledge of itself, though led thereto by its own act: because it is itself that it knows since it loves itself, as he says in the same passage. For a thing can be called self-evident in two ways, either because we can know it by nothing else except itself, as first principles are called self-evident; or because it is not accidentally knowable, as color is visible of itself, whereas substance is visible by its accident.
Reply to Objection 2: The essence of an angel is an act in the genus of intelligible things, and therefore it is both intellect and the thing understood. Hence an angel apprehends his own essence through itself: not so the human mind, which is either altogether in potentiality to intelligible things---as is the passive intellect---or is the act of intelligible things abstracted from the phantasms---as is the active intellect.
Reply to Objection 3: This saying of the Philosopher is universally true in every kind of intellect. For as sense in act is the sensible in act, by reason of the sensible likeness which is the form of sense in act, so likewise the intellect in act is the object understood in act, by reason of the likeness of the thing understood, which is the form of the intellect in act. So the human intellect, which becomes actual by the species of the object understood, is itself understood by the same species as by its own form. Now to say that in "things without matter the intellect and what is understood are the same," is equal to saying that "as regards things actually understood the intellect and what is understood are the same." For a thing is actually understood in that it is immaterial. But a distinction must be drawn: since the essences of some things are immaterial---as the separate substances called angels, each of which is understood and understands, whereas there are other things whose essences are not wholly immaterial, but only the abstract likenesses thereof. Hence the Commentator says (De Anima iii) that the proposition quoted is true only of separate substances; because in a sense it is verified in their regard, and not in regard of other substances, as already stated (Reply OBJ 2).
Objection 1: It would seem that our intellect knows the habits of the soul by their essence. For Augustine says (De Trin. xiii, 1): "Faith is not seen in the heart wherein it abides, as the soul of a man may be seen by another from the movement of the body; but we know most certainly that it is there, and conscience proclaims its existence"; and the same principle applies to the other habits of the soul. Therefore the habits of the soul are not known by their acts, but by themselves.
Objection 2: Further, material things outside the soul are known by their likeness being present in the soul, and are said therefore to be known by their likenesses. But the soul's habits are present by their essence in the soul. Therefore the habits of the soul are known by their essence.
Objection 3: Further, "whatever is the cause of a thing being such is still more so." But habits and intelligible species cause things to be known by the soul. Therefore they are still more known by the soul in themselves.
On the contrary, Habits like powers are the principles of acts. But as is said (De Anima ii, 4), "acts and operations are logically prior to powers." Therefore in the same way they are prior to habits; and thus habits, like the powers, are known by their acts.
I answer that, A habit is a kind of medium between mere power and mere act. Now, it has been said (A[1]) that nothing is known but as it is actual: therefore so far as a habit fails in being a perfect act, it falls short in being of itself knowable, and can be known only by its act; thus, for example, anyone knows he has a habit from the fact that he can produce the act proper to that habit; or he may inquire into the nature and idea of the habit by considering the act. The first kind of knowledge of the habit arises from its being present, for the very fact of its presence causes the act whereby it is known. The second kind of knowledge of the habit arises from a careful inquiry, as is explained above of the mind (A[1]).
Reply to Objection 1: Although faith is not known by external movement of the body, it is perceived by the subject wherein it resides, by the interior act of the heart. For no one knows that he has faith unless he knows that he believes.
Reply to Objection 2: Habits are present in our intellect, not as its object since, in the present state of life, our intellect's object is the nature of a material thing as stated above (Q[84], A[7]), but as that by which it understands.
Reply to Objection 3: The axiom, "whatever is the cause of a thing being such, is still more so," is true of things that are of the same order, for instance, of the same kind of cause; for example, we may say that health is desirable on account of life, and therefore life is more desirable still. But if we take things of different orders the axiom is not true: for we may say that health is caused by medicine, but it does not follow that medicine is more desirable than health, for health belongs to the order of final causes, whereas medicine belongs to the order of efficient causes. So of two things belonging essentially to the order of the objects of knowledge, the one which is the cause of the other being known, is the more known, as principles are more known than conclusions. But habit as such does not belong to the order of objects of knowledge; nor are things known on account of the habit, as on account of an object known, but as on account of a disposition or form whereby the subject knows: and therefore the argument does not prove.
Objection 1: It would seem that our intellect does not know its own act. For what is known is the object of the knowing faculty. But the act differs from the object. Therefore the intellect does not know its own act.
Objection 2: Further, whatever is known is known by some act. If, then, the intellect knows its own act, it knows it by some act, and again it knows that act by some other act; this is to proceed indefinitely, which seems impossible.
Objection 3: Further, the intellect has the same relation to its act as sense has to its act. But the proper sense does not feel its own act, for this belongs to the common sense, as stated De Anima iii, 2. Therefore neither does the intellect understand its own act.
On the contrary, Augustine says (De Trin. x, 11), "I understand that I understand."
I answer that, As stated above (AA[1],2) a thing is intelligible according as it is in act. Now the ultimate perfection of the intellect consists in its own operation: for this is not an act tending to something else in which lies the perfection of the work accomplished, as building is the perfection of the thing built; but it remains in the agent as its perfection and act, as is said Metaph. ix, Did. viii, 8. Therefore the first thing understood of the intellect is its own act of understanding. This occurs in different ways with different intellects. For there is an intellect, namely, the Divine, which is Its own act of intelligence, so that in God the understanding of His intelligence, and the understanding of His Essence, are one and the same act, because His Essence is His act of understanding. But there is another intellect, the angelic, which is not its own act of understanding, as we have said above (Q[79], A[1]), and yet the first object of that act is the angelic essence. Wherefore although there is a logical distinction between the act whereby he understands that he understands, and that whereby he understands his essence, yet he understands both by one and the same act; because to understand his own essence is the proper perfection of his essence, and by one and the same act is a thing, together with its perfection, understood. And there is yet another, namely, the human intellect, which neither is its own act of understanding, nor is its own essence the first object of its act of understanding, for this object is the nature of a material thing. And therefore that which is first known by the human intellect is an object of this kind, and that which is known secondarily is the act by which that object is known; and through the act the intellect itself is known, the perfection of which is this act of understanding. For this reason did the Philosopher assert that objects are known before acts, and acts before powers (De Anima ii, 4).
Reply to Objection 1: The object of the intellect is something universal, namely, "being" and "the true," in which the act also of understanding is comprised. Wherefore the intellect can understand its own act. But not primarily, since the first object of our intellect, in this state of life, is not every being and everything true, but "being" and "true," as considered in material things, as we have said above (Q[84], A[7]), from which it acquires knowledge of all other things.
Reply to Objection 2: The intelligent act of the human intellect is not the act and perfection of the material nature understood, as if the nature of the material thing and intelligent act could be understood by one act; just as a thing and its perfection are understood by one act. Hence the act whereby the intellect understands a stone is distinct from the act whereby it understands that it understands a stone; and so on. Nor is there any difficulty in the intellect being thus potentially infinite, as explained above (Q[86], A[2]).
Reply to Objection 3: The proper sense feels by reason of the immutation in the material organ caused by the external sensible. A material object, however, cannot immute itself; but one is immuted by another, and therefore the act of the proper sense is perceived by the common sense. The intellect, on the contrary, does not perform the act of understanding by the material immutation of an organ; and so there is no comparison.
Objection 1: It would seem that the intellect does not understand the act of the will. For nothing is known by the intellect, unless it be in some way present in the intellect. But the act of the will is not in the intellect; since the will and the intellect are distinct. Therefore the act of the will is not known by the intellect.
Objection 2: Further, the act is specified by the object. But the object of the will is not the same as the object of the intellect. Therefore the act of the will is specifically distinct from the object of the intellect, and therefore the act of the will is not known by the intellect.
Objection 3: Augustine (Confess. x, 17) says of the soul's affections that "they are known neither by images as bodies are known; nor by their presence, like the arts; but by certain notions." Now it does not seem that there can be in the soul any other notions of things but either the essences of things known or the likenesses thereof. Therefore it seems impossible for the intellect to known such affections of the soul as the acts of the will.
On the contrary, Augustine says (De Trin. x, 11), "I understand that I will."
I answer that, As stated above (Q[59], A[1]), the act of the will is nothing but an inclination consequent on the form understood; just as the natural appetite is an inclination consequent on the natural form. Now the inclination of a thing resides in it according to its mode of existence; and hence the natural inclination resides in a natural thing naturally, and the inclination called the sensible appetite is in the sensible thing sensibly; and likewise the intelligible inclination, which is the act of the will, is in the intelligent subject intelligibly as in its principle and proper subject. Hence the Philosopher expresses himself thus (De Anima iii, 9)---that "the will is in the reason." Now whatever is intelligibly in an intelligent subject, is understood by that subject. Therefore the act of the will is understood by the intellect, both inasmuch as one knows that one wills; and inasmuch as one knows the nature of this act, and consequently, the nature of its principle which is the habit or power.
Reply to Objection 1: This argument would hold good if the will and the intellect were in different subjects, as they are distinct powers; for then whatever was in the will would not be in the intellect. But as both are rooted in the same substance of the soul, and since one is in a certain way the principle of the other, consequently what is in the will is, in a certain way, also in the intellect.
Reply to Objection 2: The "good" and the "true" which are the objects of the will and of the intellect, differ logically, but one is contained in the other, as we have said above (Q[82], A[4], ad 1; Q[16], A[4], ad 1); for the true is good and the good is true. Therefore the objects of the will fall under the intellect, and those of the intellect can fall under the will.
Reply to Objection 3: The affections of the soul are in the intellect not by similitude only, like bodies; nor by being present in their subject, as the arts; but as the thing caused is in its principle, which contains some notion of the thing caused. And so Augustine says that the soul's affections are in the memory by certain notions.