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.