St. Thomas Aquinas's Philosophy of Teaching in a Nutshell

Just as a person may be cured in a twofold manner, through the operation of nature alone or through nature with the aid of medicine, so there is a twofold manner of acquiring knowledge, the one when the natural reason of itself comes to a knowledge of the unknown, which is called "discovery," the other when someone extrinsically gives aid to the natural reason, which is called "instruction." Now, in those things which are done by nature and art, art works in the same way and by the same means that nature does, for just as nature in one suffering from cold induces health by warming him, so does the doctor. Hence, art is said to imitate nature. Similarly, it happens in the acquisition of knowledge that the one teaching leads another to a knowledge of the unknown in the same way as he (the learner) would lead himself to a cognition of an unknown in discovery. Now, the process of reason in one who arrives at a cognition of an unknown in discovery is the application of general, self-evident principles to definite matters, and proceeding from them to particular conclusions and from these to others. Hence, and according to this, one man is said to teach another because the teacher proposes to another by means of symbols the discursive process which he himself goes through by natural reason, and thus the natural reason of the pupil comes to a cognition of the unknown through the aid of what is proposed to him as with the aid of instruments. As, then, a doctor is said to cause health in a sick person through the operation of nature, so man is said to cause knowledge in another through the operation of the learner's natural reason—and this is to teach.

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Source: St. Thomas Aquinas, De Magistro, trans. by Mary Helen Mayer, a. 1.