18 THE PRINCIPLES OF SCIENCE. 



Expression of Identity and Difference. 



To denote the relation of sameness or identity I unhesi- 

 tatingly adopt the sign = , so long used by mathematicians 

 to denote equality. This symbol was originally appropri- 

 ated by Robert Recorde in his 'Whetstone of Wit/ to 

 avoid the tedious repetition of the words 'is equal to': 

 and he chose a pair of parallel lines, because no two things 

 can be more equal l . The meaning of the sign has how- 

 ever been gradually extended beyond that of common 

 equality ; mathematicians have themselves used it to 

 indicate equivalency of operations. The force of analogy 

 has been so great that writers in all other branches of 

 science have more or less employed the same sign. The 

 philologist indicates by it equivalency of meaning of words : 

 chemists adopt it to signify the identity in kind and 

 equality in weight of the elements which form two different 

 compounds. Not a few logicians, for instance Ploucquet, 

 Condillac m , George Bentham 11 , Boole, have employed it 

 as the copula of propositions. Prof, de Morgan declined 

 to use it for this purpose, but still further extended its 

 meaning so as to include the equivalencyof a proposition 

 with the premises from which it can be inferred , and 

 Herbert Spencer has applied it in a like manner P. 



Many persons may think that the choice of a symbol is 

 a matter of slight importance or of mere convenience, but 

 I hold that the common use of this sign = in so many 

 different meanings is really founded upon a generalisation 



1 Hallam's ' Literature of Europe,' First Ed. vol. ii. p. 444. 

 m Condillac, 'Langue des Calculs,' p. 157. 



n 'Outline of a New System of Logic,' London, 1827, pp. 133, &c. 

 o 'Formal Logic,' pp. 82, 106. In his later work, 'The Syllabus of a 

 New System of Logic,' he discontinued the use of the sign. 

 P ' Principles of Psychology,' Second Ed., vol. ii. pp. 54, 55. 



