22 THE PRINCIPLES OF SCIENCE. 



We may also endow with special meanings the letter- 

 terms A, B and C, and the process of inference will never 

 be false. Thus let the sign *c mean 'is height of/ and 

 let 



A = Snowdon, 



B = Highest mountain in England or Wales, 

 C = 3590 feet; 



then it obviously follows that since '3590 feet is the 

 height of Snowdon,' and ' Snowdon = the highest mountain 

 in England or Wales,' then '3590 feet is the height of the 

 highest mountain in England or Wales.' 



One result of this general process of inference is that 

 we may in any aggregate or complex whole replace any 

 part by its equivalent without altering the whole. To 

 alter is to make a difference, but if in replacing a part I 

 make no difference, there is no alteration of the whole. 

 Many inferences which have been very imperfectly in- 

 cluded in logical formulae at once follow. I remember the 

 late Prof, de Morgan remarking that all Aristotle's logic 

 could not prove that * Because a horse is an animal, the 

 head of a horse is the head of an animal.' I conceive that 

 this amounts merely to replacing in the complete notion 

 head of a horse, the term ' horse' by its equivalent some 

 animal or an animal. Similarly, since 



The Lord Chancellor = The Speaker of the House of 



Lords, 

 it follows that 



The death of the Lord Chancellor = The death of the 



Speaker of the House of Lords ; 



and any event, circumstance or thing which stands in a 

 certain relation to the one will stand in like relation to 

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

 his Areopagitica, ' Who kills a man, kills a reasonable crea- 

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



God's image = man = some reasonable creature, 



