OF KNOWLEDGE. 391 



principles had fixed, and were thus forced to introduce 

 new conceptions. These new conceptions have not only 

 opened out in their application vast regions of natural 

 and historical knowledge, but have also tended to change 

 our ideas regarding its nature, leading up to a new 

 theory of knowledge and novel solutions of the ever- 

 lasting problem. It may be useful to consider some- 

 what more in detail some of the more important steps 

 by which this change has been brought about. 



Foremost in this respect stand the modern definitions 55. 



Greater 



of two terms with which older science operated, fre- precision, 

 quently unconscious of the ambiguity inherent in them. 

 These two terms are, matter and force. They have 

 been supplanted in the exact or mathematical sciences 

 by two other terms, viz., mass and energy, 1 which are 

 capable of strict definition as measurable quantities 

 in time and space. Upon them is built up the purely 

 mechanical explanation of things and phenomena. It 

 is true that in those natural sciences which deal with 

 the individual things of nature we cannot yet discard 

 the older terms, matter and force. But this accord- 

 ing to an opinion which can neither be proved nor 

 disproved only shows that where they have to be 

 employed, as when we, for instance, deal with chemical 



1 Some thinkers would prefer to 

 say Mass and Motion, and to define 

 energy in terms of Mass (or Inertia) 

 and Velocity (or rate of motion). 

 If this is done, it is evident that 

 phenomena in which mechanical 

 motion does not primarily present 

 itself must be translated into these 

 mechanical terms before they can 

 be treated with exactitude. On 

 the other side, the leaders of the 



energetic philosophy abroad, with 

 Prof. Ostwald at their head, con- 

 ceive of energy as a fundamental 

 quantity possessing two distinct 

 factors, that of quantity (capacity) 

 and that of intensity. With them 

 mechanical energy is only one form 

 of energy, and the term is conceived 

 also to embrace non-mechanical 

 (psychical) forms of energy. 



