712 



SCIENCK 



[N. S. Vol. IX. No. 229. 



3IENTAL FATIGUE. 



The purpose of this article is to give a 

 preliminary report of some experiments' on 

 mental fatigue made by the writer. It is 

 expected that they will later be presented 

 in detail, and accordingly only the method 

 and theoretical conclusions will be now 

 stated. 



Mental fatigue may mean either the fact 

 of incompetency to do certain mental work 

 or a feeling of incompetency which parallels 

 the fact or the feeling or feelings denoted 

 by our common expressions ' mentally tired,' 

 ' mentally exhausted.' Among the conclu- 

 sions to which the experiments have led are 

 the following : first, that the fact of incom- 

 petency is not what it has been supposed to 

 be ; second, that there is no pure feeling of 

 incompetency which parallels it and is its 

 sign, that consequently the mental states 

 ordinarily designated by the phrases men- 

 tioned are not states made up of such a 

 feeling of incompetency, but are very 

 complex affairs; and third, that these mental 

 states are in no sense parallels or measures 

 of the decrease in ability to do mental 

 work. 



We have been accustomed to think of 

 mental work in terms of mechanics. The 

 mind has been supposed to lose its power to 

 work as a rubber ball loses its power to 

 bound. As the ball rebounds to a lesser 

 and lesser height so the mind has been sup- 

 posed to think with less and less vigor. We 

 have talked as if sleep charged the mind 

 with mental energy as a current might 

 charge a storage- battery with electricity and 

 that then the mind had this stock to spend. 

 As it spent it, it could exert less and less 

 energy in its thinking. One could easily 

 show the impropriety of such views by 

 demonstrating the inconceivability that the 

 complexity of mental action should fit so 

 simple a scheme, but it is also useful to 

 show the same thing by proof that in the 

 case of certain people the mind does not lose 



its power to do work from having done 

 large amounts of it. My experiments show 

 in certain individuals no decrease in 

 amount, speed or accuracy of work in the 

 evenings of days of hard mental work over 

 mornings or in periods immediately follow- 

 ing prolonged mental work over periods pre- 

 ceding it. 



So far as these and many other experi- 

 ments go they all agree in denying that 

 the cause for a decreased amount of mental 

 work is such a simple lessening of some one 

 factor, mental energy or whatever one cares 

 to call it. They would afiirm, on the con- 

 trary, that we did less work when tired, 

 not because this stock of mental energy was 

 running low, but because ideas of stopping, 

 of ' taking it easj',' of working intermit- 

 tently came in and were not inhibited ; be- 

 cause feelings of boredom led to their con- 

 sequences of leaning back in one's chair, 

 looking at the clock, etc.; because a certain 

 feeling of physical strain weakened one's 

 impulse to read, write or translate ; because 

 sleepiness clouded our mental vision ; be- 

 cause headaches or eye-aches tended natu- 

 rally to inhibit the processes which caused 

 them, etc., etc. 



As to the pure feeling of incompetency I 

 fail utterly to find it in myself or to get any 

 intelligible account of it from others. After 

 one separates out from the feelings of men- 

 tal fatigue the factors just mentioned, espe- 

 cially the feelings of physical pain and 

 strain, the feelings of mental nausea at cer- 

 tain ideas, and the feeling of sleepiness, I 

 do not think that he will find anything left 

 that is worth naming. 



That the feelings of fatigue which we do 

 have are not proportionate concomitants 

 with the decreasing ability to do mental 

 work is shown by the fact that all the per- 

 sons in our experiments reported a large 

 measure of such feelings in cases where their 

 mental work was quite up to the average. 

 In general a comparison of the introspective 



