Thomas Aquinas

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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 ”

“ 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 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. ”

“ 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. ”

“ 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. ”