In the first place, the maxim cannot mean that we learn to do by every kind of doing. The kind of doing by which the young man hoped to learn medicine and surgery was ridiculed centuries ago; no one in our day would advocate mere blind doing as a means of learning. The maxim must refer to doing guided by an intelligent will. The doing must be guided by thinking that is based upon correct and reliable data or premises.
Again, the maxim cannot mean that we learn one thing by doing another. The maxim was emphasized in protest against the absurdity of some of our methods of teaching. It may happen that the learner accidentally discovers one thing while seeking to find out some other thing; to expect that this shall always be the case is to invite disappointment. For instance, pupils do not learn to spell while studying books if attention is absorbed in the meaning, and is not drawn, in separate exercises, to the correct orthography of words that are apt to be misspelled.
There is a third limitation to the maxim on the side of attention. How, for instance, is the art of writing acquired? It is undoubtedly true that a boy cannot learn to write without himself writing; it is equally true that he is not always learning or improving in penmanship while he is practising with his pen upon paper. From the teacher or the copy he gets a concept of the letters to be made. The first efforts at imitation are fraught with defects. The pupil must clearly recognize wherein he failed, and earnestly strive to remedy the defects, if the next attempt is to be an improvement. The maxim, if here applied, must mean that the pupil learns to do by continually doing, as nearly as he can, the thing to be done. With each step of progress, his concept of the form of the letters and how to make them becomes more accurate; or, in other words, his power and skill keep pace with his knowledge. Finally, after much practice, the nerves and muscles which control the act of writing are properly co-ordinated; the habit of writing with ease is acquired; the process becomes largely subconscious, if not altogether automatic. The learner has at length reached the stage in which his attention is no longer concentrated upon the form and beauty of the letters, but rather upon the thought to be expressed, and it is quite possible that henceforth his chirography will grow more illegible the more he writes. Of course, he is now learning the art of composing by composing; but he has ceased to learn in the direction of his handwriting by writing, because the attention is riveted upon something else. Even before the subconscious stage is reached, practice, if too long continued, may exhaust the powers of attention, and doing can no longer issue in learning by reason of fatigue.