When a man uses his muscles, after a time comes the feeling called fatigue—­a sensation always referred to the muscles, and due most probably to the deposit in the tissues of certain substances formed during motor activity.  Warned by this weariness, the man takes rest—­may indeed be forced to do so; but, unless I am mistaken, he who is intensely using the brain does not feel in the common use of it any sensation referable to the organ itself which warns him that he has taxed it enough.  It is apt, like a well-bred creature, to get into a sort of exalted state under the stimulus of need, so that its owner feels amazed at the ease of its processes and at the sense of wide-awakefulness and power that accompanies them.  It is only after very long misuse that the brain begins to have means of saying, “I have done enough;” and at this stage the warning comes too often in the shape of some one of the many symptoms which indicate that the organ is already talking with the tongue of disease.

I do not know how these views will be generally received, but I am sure that the personal experience of many scholars will decide them to be correct; and they serve to make clear why it is that men may not know they are abusing the organ of thought until it is already suffering deeply, and also wherefore the mind may not be as ruthlessly overworked as the legs or arms.

Whenever I have closely questioned patients or men of studious habits as to this matter, I have found that most of them, when in health, recognized no such thing as fatigue in mental action, or else I learned that what they took for this was merely that physical sense of being tired, which arises from prolonged writing or constrained positions.  The more, I fancy, any healthy student reflects on this matter the more clearly will he recognize this fact, that very often when his brain is at its clearest, he pauses only because his back is weary, his eyes aching, or his fingers tired.

This most important question, as to how a man shall know when he has sufficiently tasked his brain, demands a longer answer than I can give it here; and, unfortunately, there is no popular book since Ray’s clever and useful “Mental Hygiene,” and Feuchtersleben’s “Dietetics of the Soul,” both out of print, which deals in a readable fashion with this or kindred topics.[1] Many men are warned by some sense of want of clearness or ease in their intellectual processes.  Others are checked by a feeling of surfeit or disgust, which they obey or not as they are wise or unwise.  Here, for example, is in substance the evidence of a very attentive student of his own mental mechanism, whom we have to thank for many charming products of his brain.  Like most scholars, he can scarcely say that he ever has a sense of “brain-tire,” because cold hands and feet and a certain restlessness of the muscular system drive him to take exercise.  Especially when working at night, he gets after a time a sense of disgust at the work he is doing.  “But sometimes,” he adds, “my brain gets going, and is to be stopped by none of the common plans of counting, repeating French verbs, or the like.”  A well-known poet describes to me the curious condition of excitement into which his brain is cast by the act of composing verse, and thinks that the happy accomplishment of his task is followed by a feeling of relief, which shows that there has been high tension.