84 THE FOUNDATIONS OF SCIENCE 



step, though between them is fixed an impassable 

 gulf, across which there is no possibility of interaction. 



The second theory is that of the materialist, who 

 holds that the physical changes are the sole reality, 

 and that consciousness is merely a bye-product of 

 atomic and molecular rearrangements. 



The third theory is called idealism, and, according 

 to it, mind is the only ultimate reality, and matter 

 is one of the manifestations of some finite or infinite 

 mind. 



Finally, there is another dualistic theory which 

 regards mind and matter as distinct, but accepts the 

 common-sense view that our minds act on and are 

 acted on by matter, leaving undetermined as yet the 

 precise mode of interaction. 



It is beyond our present purpose to discuss these 

 metaphysical theories. 



We are now in a position to consider in more detail 

 the theory of knowledge, to ask, in fact, how know- 

 ledge becomes possible, whether in one way or in 

 more than one. Here we are on debatable, or, at all 

 events, debated, ground, and not all philosophers will 

 agree with the statements in the next paragraphs. 



One kind of knowledge which we dealt with in the 

 chapters on physics and biology may be termed 

 knowledge by experience. It may be sub-divided 

 into knowledge by acquaintance and knowledge by 

 description. Knowledge by acquaintance may be 



