IV.] DEDUCTIVE REASONING. 61 



is abridged in common language by the ellipsis of the 

 circumstances which are not of immediate importfince. 



Instead of all the proposition.'? being exactly of the same 

 kind as in the last example, we may have a series of 

 prenn'ses of various cliaracter ; for instance. 



Common salt is sodium chloride, (i) 



Sodium chloride crystallizes in a cubical form, (2) 

 \\'hat crystallizes in a cubical furm dops 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 tlius, 

 A = Conmion salt, 

 B = Sodium chloride, 

 C = Crystallizing in a cubiccil form, 

 D = ro.ssessiiig the power of double refraction, 

 we may state the jiremises in the forms 



A=B, (I) 



B = BC, (2) 



C - Cd. (3) 



Substituting by (3) in (2) and then by (2) as thus altered 

 in (i) we obtain 



A = BCd, (4) 



which is a more precise version of the common conclu.sion. 

 We often meet with a series of propositions describing 

 the qualities or circumstances of the one same thing, and 

 we may combine them all into one proposition by the 

 process of substitution. This case is, in fact, that which 

 Dr. Thomson has called "Immediate Inference by the 

 sum of several predicates," and his example will serve my 

 purpose well.^ He describes copper as "A metal — of a 

 red colour — and disagreeable smell — and taste — all the 

 preparations of which are poisonmis — which is highly 

 malleable — ductile — and tenacious — with a specific gravity 

 of about 8.83." If we assign the letter A to copper, and the 

 succeeding letters of the alpha) i(!t in succession to the series 

 of predicates, we have nine distinct statements, of the form 



A = AB (i) A = AC (2) A = AD (3) A = AK (9). 



We can readily combine these propositions intu one by 



' An Outline of the Neces^nnj Laws of Thounht, Filtli Ed. ]). 161 



