26 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



the theory aside and sets up a new one in its place. But here 

 the situation was different. For the theory which was negated 

 by the experiment was at the very foundation of the whole 

 physical structure; if it were to be thrown aside, most of 

 physical theory would have to be discarded. Physics was 

 thrown into turmoil. The problem of the future of physics 

 now seemed to lie in abandoning the entire structure and 

 starting again from the ground. 



Again it is not important to solve the problem here raised. 

 What should be pointed out is that physics immediately 

 became self-critical. And in becoming self-critical it became 

 philosophical. For the problems which are raised by the 

 theory of relativity are unquestionably philosophical in char- 

 acter. They have to do with the nature of time, space, and 

 motion; with the assumptions implicit in the method of 

 measurement; with the applicability of geometrical concepts 

 to the world; with the very meaning of "truth," 'theory," 

 "law," and other logical concepts. For example, if the re- 

 sults of this experiment are to be accepted, there seems to 

 be a peculiar interrelationship of space and time. This is 

 popularly, but very inaccurately, expressed by saying that 

 time turns out to be a fourth dimension of space. What is 

 meant is that space and time are shown to constitute a 

 four-dimensional system, of which three dimensions are 

 spatial and one dimension is temporal. In other words, in 

 order to locate a happening completely one must specify 

 not only where it occurs but also when it occurs. But, sur- 

 prisingly enough, one cannot tell when it occurs without 

 knowing at the same time where it occurs, nor can one tell 

 where it occurs without knowing when it occurs — hence the 

 puzzling interrelation of space and time which has been one 

 of the essential problems of the philosophy of science. Here 

 again what should be pointed out is merely that the physi- 

 cist, when he turns his attention to such problems as that 

 mentioned above, ceases to be a pure physicist and becomes 

 a philosopher of physics. This does not mean that he must 

 cease to be a physicist, but it does mean that he approaches 



