St. Thomas explains that there are two ways of learning

  The better is the way of independent investigation, which he calls "discovery." It is remarkably illustrated in the exploits of gifted children who teach themselves to read or, like the three-year-old Mozart, to play a musical instrument before having had any instruction. We all employ this method in less spectacular fashion when we acquire some store of knowledge or some skill through our own experience and effort. This procedure not only manifests greater intellectual power in the learner, St. Thomas thought, but is also more perfect. For we learn in this case through an immediate contact with the realities in question, whereas, when we are taught, the teacher’s "signs" (generally verbal ones ―illustrations, explanations, etc.) intervene and, at best, point us toward those realities. It is a rare talent, nonetheless, that can wholly dispense with a teacher’s help and to do so is, in any case, time consuming. So that the chief value of this second way of learning, that is to say, learning-through-teaching, is one of economy. Most men would have neither the leisure nor the courage to learn all they need to know if teachers did not ease and accelerate the process for them.  St. Thomas Aquinas and Education by John W. Donohue, S.J. (Random House, 1968).

  St. Thomas’s recommendation makes clear the primacy of that personal way of discovery. The teacher, he says, should pattern his method after the one naturally used, when the student learns by himself. This is based on the general principle that whenever an effect can be produced either by natural process or by artificial method, the method should as much as possible be the same as that of nature. The great medieval scholar stresses the difference which exists between a work which can only be produced by artificial means (for instance the construction of a machine) and a work which can also be produced by nature (for instance the growth of a plant). Teaching belongs to the second category. A teacher will strive to assist and not replace the natural energies of his student. He will not be like an engineer but more like a farmer who helps the growth of the tree through watering, pruning, weeding. The principal cause of growth is the natural vigor of the tree, the farmer is only assisting it.

  We have been victims since Descartes of an exaggerated emphasis on "methods." The student has been viewed with a mechanistic perspective. Teaching is synonymous with trying to cram information into the student as if he was a passive machine instead of a living mind. St. Thomas is quite opposed to this caricature of true teaching. He explains that the procedures of teaching will be most effective when patterned after those of independent search. A good teacher will make the difference between learning by discovery and learning by instruction as narrow as possible. In other words he will assist his students in their learning so that their experience will seem to them a discovery. Some great teachers really have this gift. Their classes are so good that you‘re constantly finding out new things and getting excited about them. The challenge and the enthusiasm of learning are kept alive by the evident love that these men have for their subject. The teachers cannot but communicate to their students who come out of the classroom with a desire to learn more.

  Maybe we could use another illustration from St. Thomas to understand better his conception of the teacher’s role:

  The example is that of a physician ministering to a man brought down by an infection. In many such cases, if the patient went unattended, his body would mobilize its restorative forces and eventually heal itself. The art of the teacher is like that of this internist, since all the teacher can hope to do is to strengthen the student’s resources and facilitate their exercise. The import of the analogy is clear enough whether it is medically accurate or not. Whenever true learning occurs, its principal cause is the learner himself. In St. Thomas’s terminology the teacher is called a secondary and instrumental cause ―helpful but not indispensable. He cannot transfer his own knowledge to the student but only help him achieve similar learning for himself.  St. Thomas Aquinas and Education by John W. Donohue, S.J. (Random House, 1968).

  We see that we are far removed from the modern conception of teaching where the teacher exclusively relies on artificial means (workbooks and other devices) and slowly loses sight of his true function: to stimulate the living mind of the student, to guide the development of his intellect. The emphasis on "measuring" education with grades obtained through tests is partly a result of this false philosophy. Knowledge according to St. Thomas is a quality of the living mind and not a quantity of memorized information