ii2 SCIENTIFIC METHOD IN PHILOSOPHY 



thing will be one whose states at all times are ideal. 

 Ideal appearances, states, and things, since they are 

 calculated, must be functions of actual appearances, states, 

 and things ; in fact, ultimately, they must be functions of 

 actual appearances. Thus it is unnecessary, for the 

 enunciation of the laws of physics, to assign any reality to 

 ideal elements : it is enough to accept them as logical 

 constructions, provided we have means of knowing how 

 to determine when they become actual. This, in fact x we 

 have with some degree of approximation ; the starry 

 heaven, for instance, becomes actual whenever we choose 

 to look at it. It is open to us to believe that the ideal 

 elements exist, and there can be no reason for ^believing 

 this ; but unless in virtue of some a priori law we cannot 

 know it, for empirical knowledge is confined to what we 

 actually observe. 



(2) The three main conceptions of physics are space, 

 time, and matter. Some of the problems raised by the 

 conception of matter have been indicated in the above 

 discussion of " things." But space and time also raise 

 difficult problems of much the same kind, namely, 

 difficulties in reducing the haphazard untidy world of 

 immediate sensation to the smooth orderly world of 

 geometry and kinematics. Let us begin with the con- 

 sideration of space. 



People who have never read any psychology seldom 

 realise how much mental labour has gone into the con- 

 struction of the one all-embracing space into which all 

 sensible objects are supposed to fit. Kant, who was 

 unusually ignorant of psychology, described space as " an 

 infinite given whole," whereas a moment's psychological 

 reflection shows that a space which is infinite is not given, 

 while a space which can be called given is not infinite. 

 What the nature of " given " space really is, is a difficult 



