602 PHILOSOPHICAL THOUGHT. 



retical views which have opened out new vistas of 

 scientific thought, are tempted to apply their principles 

 and habits of reasoning to a larger region than that in 

 which they were originally at home, and this without a pre- 

 vious critical examination as to the applicability or valid- 

 ity of the notions they have employed. The proof of the 

 correctness of these notions lies for them in the practical 

 success which they have achieved in a more or less re- 

 stricted field. The temptation to extend the use of any 

 serviceable tool or instrument applies as much to logical 

 as to mechanical devices, and we find this to be the case 

 as much in science and philosophy as in practical life, in 

 the arts, industries, and business. And this temptation, 

 so far as abstract thought is concerned, is greater where 

 we employ ideas and terms which have a double mean- 

 ing, pointing, as it were, in two directions. This is the 

 case pre-eminently in the natural sciences as distinguished 

 from the mathematical sciences on the one side and from 

 the mental sciences on the other. All naturalists, includ- 

 ing also eminent representatives of the medical profession, 

 deal with fundamental notions which are applicable to 

 phenomena lying on the border- land of the purely physical 

 and the purely mental, of the outer and inner world, of 

 inanimate and animated nature. The words or terms also 

 by which these notions are expressed in language were 

 originally derived from that border-land, and are, in their 

 full and direct meaning, only applicable there. Those 

 who successfully operate with these notions are frequently 

 led to believe that they possess in them a master-key 

 which unlocks both worlds, affording a view into the 

 external and the internal alike. 



