reasons, the brain should never be used during, or immediately after, violent muscular exercise.  For the motor nerves are in this respect on a par with the sensory nerves; the pain felt when a limb is wounded has its seat in the brain; and, in the same way, it is not really our legs and arms which work and move,—­it is the brain, or, more strictly, that part of it which, through the medium of the spine, excites the nerves in the limbs and sets them in motion.  Accordingly, when our arms and legs feel tired, the true seat of this feeling is in the brain.  This is why it is only in connection with those muscles which are set in motion consciously and voluntarily,—­in other words, depend for their action upon the brain,—­that any feeling of fatigue can arise; this is not the case with those muscles which work involuntarily, like the heart.  It is obvious, then, that injury is done to the brain if violent muscular exercise and intellectual exertion are forced upon it at the same moment, or at very short intervals.

What I say stands in no contradiction with the fact that at the beginning of a walk, or at any period of a short stroll, there often comes a feeling of enhanced intellectual vigor.  The parts of the brain that come into play have had no time to become tired; and besides, slight muscular exercise conduces to activity of the respiratory organs, and causes a purer and more oxydated supply of arterial blood to mount to the brain.

It is most important to allow the brain the full measure of sleep which is required to restore it; for sleep is to a man’s whole nature what winding up is to a clock.[1] This measure will vary directly with the development and activity of the brain; to overstep the measure is mere waste of time, because if that is done, sleep gains only so much in length as it loses in depth.[2]

[Footnote 1:  Of.  Welt als Wille und Vorstellung, 4th Edition.  Bk.  II. pp. 236-40.]

[Footnote:  2:  Cf. loc:  cit:  p. 275.  Sleep is a morsel of death borrowed to keep up and renew the part of life which is exhausted by the day—­le sommeil est un emprunt fait a la mort.  Or it might be said that sleep is the interest we have to pay on the capital which is called in at death; and the higher the rate of interest and the more regularly it is paid, the further the date of redemption is postponed.]

It should be clearly understood that thought is nothing but the organic function of the brain; and it has to obey the same laws in regard to exertion and repose as any other organic function.  The brain can be ruined by overstrain, just like the eyes.  As the function of the stomach is to digest, so it is that of the brain to think.  The notion of a soul,—­as something elementary and immaterial, merely lodging in the brain and needing nothing at all for the performance of its essential function, which consists in always and unweariedly thinking—­has undoubtedly