174 WHAT IS SCIENCE? 



procedure that it has been able to establish true theories 

 which may be trusted, provisionally at least, in new 

 analysis. The practical man is apt to sneer at the 

 theorist ; but an examination of any of his most firmly- 

 rooted prejudices would show at once that he himself 

 is as much a theorist as the purest and most academic 

 student ; theory is a necessary instrument of thought 

 in disentangling the amazingly complex relations of the 

 external world. But while his theories are false because 

 he never tests them properly, the theories of science are 

 continually under constant test and only survive if they 

 are true. It is the practical man and not the student 

 of pure science who is guilty of relying on extrava- 

 gant speculation, unchecked by comparison with solid 

 fact. 



^ Closely connected with the errors of false theories are 

 those which arise from false, or more often incomplete, 

 laws. Such laws are, of course, in themselves errors, 

 but they often breed errors much more serious. For 

 we have seen that the things between which laws assert 

 relations are themselves interconnected by laws ; if we 

 start with false laws we are sure to interpret our 

 experience on the wrong lines, because the things between 

 which we shall try to find laws will be such that no laws 

 can involve them. An example which we have used 

 before will make this source of error clear. The word 

 " steel " is used in all but the strictest scientific circles 

 to denote many different things ; or, in other words, 

 there is no law asserting the association of all the pro- 

 perties of all the things which are conventionally called 

 steel. Accordingly there can be no law, strictly true, 

 asserting anything about steel ; for though a law may be 

 found which is true of many kinds of steel, some kind 

 of " steel " can almost certainly be found of which it 

 is not true. If we want to find true laws involving the 

 materials all of which are conventionally termed steel, 



