Objection 1. It would seem that one man cannot teach another. For the Lord says (Matthew 22:8): "Be not you called Rabbi": on which the gloss of Jerome says, "Lest you give to men the honor due to God." Therefore to be a master is properly an honor due to God. But it belongs to a master to teach. Therefore man cannot teach, and this is proper to God.
Objection 2. Further, if one man teaches another this is only inasmuch as he acts through his own knowledge, so as to cause knowledge in the other. But a quality through which anyone acts so as to produce his like, is an active quality. Therefore it follows that knowledge is an active quality just as heat is.
Objection 3. Further, for knowledge we require intellectual light, and the species of the thing understood. But a man cannot cause either of these in another man. Therefore a man cannot by teaching cause knowledge in another man.
Objection 4. Further, the teacher does nothing in regard to a disciple save to propose to him certain signs, so as to signify something by words or gestures. But it is not possible to teach anyone so as to cause knowledge in him, by putting signs before him. For these are signs either of things that he knows, or of things he does not know. If of things that he knows, he to whom these signs are proposed is already in the possession of knowledge, and does not acquire it from the master. If they are signs of things that he does not know, he can learn nothing therefrom: for instance, if one were to speak Greek to a man who only knows Latin, he would learn nothing thereby. Therefore in no way can a man cause knowledge in another by teaching him.
On the contrary, The Apostle says (1 Timothy 2:7): "Whereunto I am appointed a preacher and an apostle... a doctor of the Gentiles in faith and truth."
I answer that, On this question there have been various opinions. For Averroes, commenting on De Anima iii, maintains that all men have one passive intellect in common, as stated above (Question 76, Article 2). From this it follows that the same intelligible species belong to all men. Consequently he held that one man does not cause another to have a knowledge distinct from that which he has himself; but that he communicates the identical knowledge which he has himself, by moving him to order rightly the phantasms in his soul, so that they be rightly disposed for intelligible apprehension. This opinion is true so far as knowledge is the same in disciple and master, if we consider the identity of the thing known: for the same objective truth is known by both of them. But so far as he maintains that all men have but one passive intellect, and the same intelligible species, differing only as to various phantasms, his opinion is false, as stated above (Question 76, Article 2).
Besides this, there is the opinion of the Platonists, who held that our souls are possessed of knowledge from the very beginning, through the participation of separate forms, as stated above (84, 3,4); but that the soul is hindered, through its union with the body, from the free consideration of those things which it knows. According to this, the disciple does not acquire fresh knowledge from his master, but is roused by him to consider what he knows; so that to learn would be nothing else than to remember. In the same way they held that natural agents only dispose (matter) to receive forms, which matter acquires by a participation of separate substances. But against this we have proved above (79, 2; 84, 3) that the passive intellect of the human soul is in pure potentiality to intelligible (species), as Aristotle says (De Anima iii, 4).
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 (106, 1; 111, 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 to Objection 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" (Psalm 102:3); so of Him is it written: "He that teacheth man knowledge" (Psalm 93:10), inasmuch as "the light of His countenance is signed upon us" (Psalm 4:7), through which light all things are shown to us.
Reply to Objection 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 to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)