THE CAUSE OF INFECTIOUS DISEASE. 231 



thing internal and exactly concurrent with the 

 effect, in the second case the cause is some- 

 thing external which neither qualitatively nor 

 quantitatively stands in any sort of congruent 

 relation with the effect. In order to eliminate 

 this double sense of the word cause and to 

 put an end to all confusion, a general agree- 

 ment as to its usage was reached in connection 

 with the discovery of the law of the conserva- 

 tion of energy by R. Mayer and his successors, 

 and this usage has generally been conformed 

 to in epistemology also. 



If potential energy or the capacity for doing 

 work is transformed into kinetic energy or 

 actual work the two are equal in quantity. 

 They pass over quantitatively into one an- 

 other, and the work appearing as effect accu- 

 rately corresponds to and is measured by the 

 initial capacity for work as cause. The true 

 and sufficient cause of any effect is always 

 something internal, something that follows 

 from the kind and amount of the initial en- 

 ergy, and from that quality and qiiantity alone 

 and entirely. This is the conception of a 

 cause, Ursache , an idea which the German 

 language can express so felicitously, while 

 other languages must paraphrase it (in Latin, 

 e. g., causa prima or princeps, to which the 

 further definition causa interna and vera or 



