INTRODUCTION. 2 3 



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 person is identical 

 in meaning with individual, it follows that 



Meeting of persons = meeting of individuals ; 

 and if assemblage = meeting, we may make a new replace- 

 ment and 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 make same wholes. 



If, for instance, exactly similar bricks be used to build 

 two houses, and they be similarly 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 be undistinguishable, 

 and would be only numerically different. It is upon this 

 principle, as we shall see, that all accurate processes of 

 measurement depend. If for a weight in a scale of 

 a balance we substitute another weight, and the equili- 

 brium remains entirely unchanged, then the weights must 

 be exactly equal. The general test of equality is substi- 

 tution. Objects are equally bright when on replacing one 

 by the other the eye perceives no difference. Two objects 

 are equal in dimensions when tested by the same gauge 

 they fit in the same manner. Generally speaking, two 

 objects are alike so far as when substituted one for another 

 no alteration is produced, and vice versa when alike no 

 alteration is produced by the substitution. 



q ' Pure Logic, or the Logic of Quality,' p. 14. 



