DEDUCTIVE REASONING. 73 



for C in (2) we can substitute its equivalent in (3). We 

 shall obtain as an intermediate result, 



A = ABC, 

 and from this the complete conclusion 



A = ABCD. (4) 



The full interpretation is that Iron is iron, metal, good 

 conductor of electricity, useful for telegraphic purposes, 

 which is abridged in common language by the ellipsis of 

 the circumstances which are not of immediate importance. 

 Instead of all the propositions being of one type, as in 

 the last example, we may have a series of premises of 

 various character ; for instance 



Common salt is sodium chloride (i) 



Sodium chloride crystallizes in a cubical form (2) 

 What crystallizes in a cubical form does not 



possess the power of double refraction ; (3) 

 it will follow that 



Common salt does not possess the power of 



double refraction. (4) 



Taking our letter-terms thus 

 A = Common salt, 

 B = Sodium chloride, 

 C = Crystallizing in a cubical form, 

 D = Possessing the power of double refraction, 

 we may state the premises in the form 



A = B, (i) 



B = BC, (2) 



C = Cd. (3) 



Substituting by (2) in (i) and by (3) in (2) we obtain 



A = BCd, (4) 



which is a more precise version of the common conclusion. 

 We often meet with a series of propositions describing 

 the qualities or circumstances of one same thing, and we 

 may if we like combine them all into one proposition 

 by the process of substitution. This case is, in fact, 



