[Footnote 2: Frederick S. Lee: "Physical Exercise from the Standpoint of Physiology," _Science_, N.S., vol. XXIX, no. 744, p. 525.] [Footnote 3: Lee: _Fatigue_. Harvey Lectures, 1905-06, p. 180.] [Footnote 4: For a summary of nervous fatigue and extensive bibliography, see Goldmark: _loc. cit._, p. 32.] The conditions of mental fatigue, however, can be by no means as simply described as those of physical fatigue. Elaborate experiments by Professor Thorndike and others tend to show that, in the strictest sense of the term, there is no such thing as mental fatigue. That is, any mental function may be performed for several hours with the most negligible decrease in the efficiency of the results attained. The subject of one experiment kept continuously for seven hours performing mental multiplications of four-place numbers by four-place numbers with scarcely any perceptible decrease in speed or accuracy in results.[1] Professor Thorndike draws from this and similar experiments the conclusion that it is practically impossible to impair the efficiency of any mental function as such. What happens when we say our mental efficiency is being impaired is rather that we _will not_ than that we _cannot_ perform any given mental function. The causes of loss of efficiency are rather competing impulses[2] than fatigue in specific mental functions. We are tired _of_ the work, not _by_ it. Continuous mental work of any given kind, writing a book, solving problems in calculus, translating French, etc., involves our being withheld from other activities, games, music, or companionship, to which by force of habit or instinct, we are diverted, and diverted more acutely the more we remain at a fixed task. That it is not mental "fatigue" so much as distraction that prevents us from persisting at work is evidenced in the longer time we can stick to work that really interests us than to tasks in which we have only a perfunctory or compulsory interest. The college student who is "too dead tired" to stay up studying trigonometry will, though in the same condition, stay up studying football strategy, rehearsing for a varsity show, or getting out the next morning's edition of his college paper. "If each man did the mental work for which he was fit, and which he enjoyed, men would work willingly much longer than they now do."[1*] The effects of mental fatigue are, when analyzed, due chiefly to the physically injurious effects that do, but do not necessarily, accompany mental work. [Footnote 1: T. Arai: _Mental Fatigue_.] [Footnote 2: Thorndike: _Educational Psychology_, Briefer Course, p. 322.] [Footnote 1*: Thorndike: _Educational Psychology_, Briefer Course, p. 326.]