OF NATURE. 603 



Further, workers on this border-land can apply the 

 rigid methods of the exact sciences only to a limited 

 extent; they are everywhere led, by observation as well 

 as reflection, into departments where rigid definition is 

 impossible ; especially the medical teacher is many times 

 obliged as Du Bois Reymond himself has honestly con- 

 fessed to teach things which he does not know. Like 

 all practical professions, the medical profession embraces 

 a totality of things, many of which are matters of con- 

 jecture rather than knowledge. Thus it comes about that 

 philosophical speculations issuing from naturalists are in 41 - 

 the same degree more intelligible to the popular mind as JS^f* 1 

 they are unsatisfactory to those who start with mechani- concepts. 

 cal or mathematical notions and habits of thought on the 

 one side, or with exclusively psychological and subjective 

 notions on the other. For the former complain that 

 the naturalist uses many words and terms not in a rigidly 

 scientific sense, and the latter complain that he deals 

 with purely psychical phenomena by analogy with exter- 

 nal processes which are not really analogous, and give 

 only a semblance of insight. 



As stated above, it has taken thousands of years 

 before such terms as matter, force, energy, potential and 

 actual, have been sufficiently cleared of their purely 

 subjective attributes to enable them to be mathe- 

 matically defined. In the literature of the naturalist, 

 the physiologist, and psychologist, these terms, however, 

 still occur in a wider sense and are indispensable, denot- 

 ing something additional and different from the restricted 

 sense in which they occur in the purely exact sciences. 

 In addition to these terms we have others like sub- 



