THE EXPLANATION OF LAWS 97 



supposition underlying the theory, one which could never 

 be observed either because it is too small or too remote 

 in the past or for some similar reason and therefore 

 does not form part of the proper subject-matter of science. 

 It is because the explanation offered by a theory is 

 always based on an analogy with laws that the distinction 

 between laws and theories has been so often overlooked. 

 The statement of the dynamical theory of gases about 

 the properties and behaviour of molecules, or of the theory 

 of evolution about the process whereby the existing 

 species of living beings came into existence, is so similar 

 to the statement, asserted by a law, about the properties 

 of actual mechanical 'systems or about the changes that 

 are proceeding in existing species that the vital difference 

 between the two is forgotten. The statement asserted 

 by a law can be proved by direct perception ; it states 

 something which can be observed and which can be the 

 subject of universal assent. The statement involved 

 in the theory cannot be proved by direct perception, for 

 it does not state anything that can be or has been observed. 

 The failure to observe this distinction and the conse- 

 quent failure to give to theories their true place in the 

 scheme of science is the cause of most of the misunder- 

 standings that are so widely prevalent concerning the 

 nature and objects of science. For it has been admitted 

 that, though the discovery of laws depends ultimately 

 not on fixed rules but on the imagination of highly 

 gifted individuals, this imaginative and personal element 

 is much more prominent in the development of theories ; 

 the neglect of theories leads directly to the neglect of 

 the imaginative and personal element in science. It 

 leads to an utterly false contrast between " materialistic " 

 science and the " humanistic " studies of literature, 

 history and art. 



