I.] INTRODUCTION. 19 



certain relation to the one will stand in like relation to the 

 other. Milton reasons in this way when he says, in liis 

 Areopagitica, " Who kills a man, kills a reasonable creature, 

 God's image." If we may suppose him to mean 



God's image = man = some reasonable creature, 

 it follows that " The killer of a man is the killer of some 

 reasonable creature," and also " The killer of God's image." 

 This replacement of equivalents may be repeated over 

 and over again to any extent. Thus if inrson is identical 

 in meaning with individital, it follows that 



Meeting of persons = meeting of individuals ; 

 nnd if as^s^fmhlarjc = meeting, we may make a new replace- 

 ment Rud show that 



Meeting of persons = assemblage of individuals. 

 We may in fact found upon this principle of substitution 

 a most general axiom in the following terms ^ : — 



Same parts samely related malie same irJioles. 

 If, for instance, exactly similar bricks and other 

 materials be used to build two houses, and they be simi- 

 larlv placed in each house, the two houses must be similar. 

 There are millions of cells in a human body, but if each 

 cell of one person were represented by an exactly similar 

 cell similarly placed in another body, the two persons 

 would beundistinguishable, and would be only numeriealhi 

 different. It is upon this principle, as we shall see, tliat 

 all accurate processes of measurement depend. If for a 

 weight in a scale of a balance we substitute another 

 weight, and the equilibrium remains entirely unchanged, 

 then the weights must be exactly equal. The general test 

 of equality is substitution. Objects are equally bright 

 when on replacing one by the other the eye perceives no 

 difference. Objects are ecjual in dimensions when tested 

 by the same gauge they tit in the same manner. Generally 

 speaking, two objects are alike so I'ar as when substituted 

 one for another no alteration is produced, and vice versa 

 when alike no alteration is produced by the substitution. 



1 Pure Logic, or (he Logic 0/ Quality, p. 14. 



c 2 



