ANALOGY A GREAT AID TO DISCOVERY. 327 



Analogy is a great aid to discovery. If we can perceive 

 in a new phenomenon a real and essential similarity to 

 others we well understand, we have advanced a great step 

 towards its true explanation, because we may then conclude 

 that the principles which govern the latter operate in the 

 former. And as there may be found in different cases all 

 degrees of similarity between two different objects, vary- 

 ing from that of complete identity to entire difference, so 

 in proportion to the degree of real similarity may we 

 safely predict of one what we know to be true of the other. 

 The analogy of logic and algebra was the basis of the dis- 

 coveries made in logic by Professor Boole ; and the simi- 

 larity between algebra and geometry shown by Descartes, 

 in the general truth that every curve or figure in space 

 represents an equation, has for many years been the chief 

 source of new mathematical methods. The analogies of 

 light and radiant heat have also led to many discoveries. 

 Immediately after Faraday, in the year 1845, had dis- 

 covered the rotation of the plane of a beam of polarised 

 light by a magnet, Wartmann found that a beam of 

 polarised heat -rays was similarly affected. Similar re- 

 marks may be made with regard to the analogies of sound 

 and light. 



The detection of difference or likeness of things dis- 

 covered, by comparison with things already known, is 

 essential to successful research, because it enables us to 

 classify a phenomenon or discovery, and approximately 

 determine its nature. One substance is distinguished 

 from another only by its difference of properties ; if two 

 different bodies existed, possessing exactly the same pro- 

 perties, we should be absolutely unable to distinguish one 

 from the other. We know that two separate portions of 

 any perfectly homogeneous liquid or gas are not identical ; 

 but as they possess no difference of properties, we cannot 



