28 PHYSICAL SCIENCE 



involved that the idea of causation is most vivid. 

 When one of the two related phenomena seems 

 to follow the other, the mind instinctively identifies 

 post hoc with propter hoc. The principle of 

 relativity has shown that there is no absolute 

 scale of time in which events may be placed in 

 order. In some cases, one observer may say that 

 A precedes B, while to another B happens first. 

 But, even if a distinction between cause and 

 effect is philosophically difficult, as a matter of 

 convenience in language it is perhaps justified. 

 When carefully examined, however, the difficulty 

 of isolating the ''cause" of any particular ''effect" 

 will be found to be insuperable. A long train 

 of circumstances has preceded the phenomenon 

 considered, and the phenomenon would not have 

 appeared had any one of those circumstances 

 been absent. Each or all of them might equally 

 well have been called the "cause." Whether 

 the idea of cause and effect represents a real 

 distinction in the hypothetical world which our 

 conceptions represent, remains, like the nature 

 and existence of that world itself, an inquiry for 

 the philosopher. 



Physical Science, then, seeks to establish 

 general rules which describe the sequence of 

 phenomena in all cases. Underlying all such 

 attempts is the belief that such an orderly 

 sequence is invariably present, could it only be 

 traced. This belief, which is the result of 

 constant experience, is known as the principle of 

 the Uniformity of Nature. In its absence no 

 organised knowledge could be obtained, and any 

 attempt to investigate phenomena would be 

 perfectly useless. Unless, to use the conven- 



