80 TWO NEW WORLDS 



Where, then, is that " unifying principle " upon 

 which we have relied to preserve our minds from 

 the intolerable complexity of the universe ? 



That such a question should be asked at all 

 shows how a mechanical view of natural pheno- 

 mena has obscured our appreciation of the reali- 

 ties underlying all human understanding. Atoms, 

 electrons, material objects generally are not realities. 

 They are our conceptions of realities which affect 

 our sensorium, constructed in our minds from 

 material supplied by our past experiences. Our 

 experiences are the only realities of which we 

 have definite evidence, and these are finally re- 

 solvable into sensations and memories of sensations. 

 By an act of faith we extend our own sphere of 

 sensation to include spheres which we perceive 

 to be similar, and we thus are enabled to see 

 with other persons' eyes and remember with other 

 persons' memories. By another act of faith we 

 postulate an " object" behind a bundle of permanent 

 or recurring sensations. These sensations are the 

 symbol of that object, the signs by which it reveals 

 its presence to us. No doubt the object contains 

 some ultimate reality, but what that ultimate reality 

 may be, what the rest of its properties are, we 

 can only faintly guess. We have only one key. In 

 ourselves we can observe both the inner reality of 

 a thing and its external and visible symbol. A 

 delicate galvanometer may show the current im- 



