We must therefore decide the question differently, by saying that the teacher causes knowledge in the learner, by reducing him from potentiality to act, as the Philosopher says (Phys. viii, 4). In order to make this clear, we must observe that of effects proceeding from an exterior principle, some proceed from the exterior principle alone; as the form of a house is caused to be in matter by art alone: whereas other effects proceed sometimes from an exterior principle, sometimes from an interior principle: thus health is caused in a sick man, sometimes by an exterior principle, namely by the medical art, sometimes by an interior principle as when a man is healed by the force of nature. In these latter effects two things must be noticed. First, that art in its work imitates nature for just as nature heals a man by alteration, digestion, rejection of the matter that caused the sickness, so does art. Secondly, we must remark that the exterior principle, art, acts, not as principal agent, but as helping the principal agent, but as helping the principal agent, which is the interior principle, by strengthening it, and by furnishing it with instruments and assistance, of which the interior principle makes use in producing the effect. Thus the physician strengthens nature, and employs food and medicine, of which nature makes use for the intended end.
Now knowledge is acquired in man, both from an interior principle, as is clear in one who procures knowledge by his own research; and from an exterior principle, as is clear in one who learns (by instruction). For in every man there is a certain principle of knowledge, namely the light of the active intellect, through which certain universal principles of all the sciences are naturally understood as soon as proposed to the intellect. Now when anyone applies these universal principles to certain particular things, the memory or experience of which he acquires through the senses; then by his own research advancing from the known to the unknown, he obtains knowledge of what he knew not before. Wherefore anyone who teaches, leads the disciple from things known by the latter, to the knowledge of things previously unknown to him; according to what the Philosopher says (Poster. i, 1): "All teaching and all learning proceed from previous knowledge."
Now the master leads the disciple from things known to knowledge of the unknown, in a twofold manner. Firstly, by proposing to him certain helps or means of instruction, which his intellect can use for the acquisition of science: for instance, he may put before him certain less universal propositions, of which nevertheless the disciple is able to judge from previous knowledge: or he may propose to him some sensible examples, either by way of likeness or of opposition, or something of the sort, from which the intellect of the learner is led to the knowledge of truth previously unknown. Secondly, by strengthening the intellect of the learner; not, indeed, by some active power as of a higher nature, as explained above (Q. 106, A. 1; Q. 111, A. 1) of the angelic enlightenment, because all human intellects are of one grade in the natural order; but inasmuch as he proposes to the disciple the order of principles to conclusions, by reason of his not having sufficient collating power to be able to draw the conclusions from the principles. Hence the Philosopher says (Poster. i, 2) that "a demonstration is a syllogism that causes knowledge." In this way a demonstrator causes his hearer to know.
Reply Obj. 1: As stated above, the teacher only brings exterior help as the physician who heals: but just as the interior nature is the principal cause of the healing, so the interior light of the intellect is the principal cause of knowledge. But both of these are from God. Therefore as of God is it written: "Who healeth all thy diseases" (Ps. 102:3); so of Him is it written: "He that teacheth man knowledge" (Ps. 93:10), inasmuch as "the light of His countenance is signed upon us" (Ps. 4:7), through which light all things are shown to us.
Reply Obj. 2: As Averroes argues, the teacher does not cause knowledge in the disciple after the manner of a natural active cause. Wherefore knowledge need not be an active quality: but is the principle by which one is directed in teaching, just as art is the principle by which one is directed in working.
Reply Obj. 3: The master does not cause the intellectual light in the disciple, nor does he cause the intelligible species directly: but he moves the disciple by teaching, so that the latter, by the power of his intellect, forms intelligible concepts, the signs of which are proposed to him from without.
Reply Obj. 4: The signs proposed by the master to the disciple are of things known in a general and confused manner; but not known in detail and distinctly. Therefore when anyone acquires knowledge by himself, he cannot be called self-taught, or be said to have his own master because perfect knowledge did not precede in him, such as is required in a master. _______________________
SECOND ARTICLE [I, Q. 117, Art. 2]
Whether Man Can Teach the Angels?
Objection 1: It would seem that men teach angels. For the Apostle says (Eph. 3:10): "That the manifold wisdom of God may be made known to the principalities and powers in the heavenly places through the Church." But the Church is the union of all the faithful. Therefore some things are made known to angels through men.