474 Edward Livingston Youmans. 



by reaching a knowledge of the true causes of insanity and 

 imbecility, we gain command of the means of their preven- 

 tion, and arrive at the principles of mental hygiene. And 

 this leads to the consideration of those wider consequences 

 to society at large which the modern method of inquiry is 

 beginning to produce. 



This is perhaps best illustrated in the establishment of 

 what may be called the law of mental limitations. The old 

 contrast between matter and mind led to the growth of an 

 all-prevalent error upon this point. To matter belongs 

 extension or limitation in space ; but mind is inextended, 

 and therefore it has been inferred to be unlimited : being 

 indefinite, it was supposed to be unbounded in its nature. 

 But force also is inextended, although rigorously limited 

 and measurable ; and as mind is nothing more nor less than 

 mental power, it must be subject to the laws of power, and 

 work within quantitative limits, like any other form of 

 force. Power, again, is but the accompaniment of material 

 change, and is hence restricted in quantity by the amount 

 of that change ; and as mind is accompanied by cerebral 

 transformation, it must have a necessary limit in the quan- 

 tity of cerebral transformation. In, therefore, considering 

 man as a being in whom mind is conditioned by a bodily 

 organism, the limitation of mental effects becomes a prac- 

 tical question of the very highest importance. 



The doctrine of the conservation of energy and the 

 mutual convertibility of the various forces is now accepted 

 as a fundamental truth of science. Nor is there any 

 ground for regarding the vital forces as an exception to 

 the principle. That the organism cannot create its own 

 force, that its energy is entirely derived from the food in- 

 gested, and which, in this point of view, is merely stored 

 force, is beyond question ; and the source being thus 

 limited, that its expenditure in one direction makes it im- 

 possible to use it in another, is equally evident. This 



