286 THE PRINCIPLES OF SCIENCE. 



Analogy as a Guide in Discovery. 



There can be no doubt that discovery is most frequently 

 accomplished by following up hints received from analogy, 

 as Jeremy Bentham remarked a . Whenever a phenomenon 

 is perceived, the first impulse of the mind is to connect it 

 with the most nearly similar phenomenon. If we could 

 ever meet a thing wholly sui generis, presenting no 

 analogy to anything else, we should be incapable of 

 investigating its nature, except by purely haphazard 

 trial. The probability of success by such a process is so 

 slight, that it is preferable to follow up the slightest clue. 

 As I have pointed out already (vol. ii. p. 24), the possible 

 modifications of condition in experiments are usually in- 

 finite in number, and infinitely numerous also are the 

 hypotheses upon which we may proceed. Now it is self- 

 evident that, however slightly superior the probability of 

 success by one course of procedure may be over another, 

 the most probable one should always be adopted first. 



The chemist having discovered what he believes to be 

 a new element, will have an infinite variety of modes of 

 treating and investigating it. If in any one of its qualities 

 the substance displays a resemblance to an alkaline metal, 

 for instance, he will naturally proceed to try whether it 

 possesses other properties common to the alkaline metals. 

 Even the apparently simplest phenomenon presents so 

 many points for notice that we have a choice at each 

 moment from among many hypotheses. 



It would be difficult to find a more instructive instance 

 of the way in which the mind is guided by analogy than 

 in the description by Sir John Herschel of the course of 

 thought by which he was led to anticipate in theory one 

 of Faraday's greatest experimental discoveries. Sir John 

 a 'Essay on Logic/ 'Works,' vol. viii. p. 276. 



