60 THE PRINCIPLES OF SCIENCE. 



so long as the vital principle of reasoning was not clearly 

 expressed. 



Immediate Inference. 



Probably the simplest of all forms of inference is that 

 which has been called Immediate Inference, because it 

 can be performed upon a single proposition. It consists 

 in joining an adjective, or other qualifying clause of the 

 same nature, to both sides of an identity, and asserting 

 the equivalence of the terms thus produced. For instance, 

 since 



Conductors of electricity = Non-electrics, 

 it follows that 



Liquid conductors of electricity = Liquid non-electrics. 

 If we suppose that 



Plants = Bodies decomposing carbonic acid, 

 it follows that 



Microscopic plants = Microscopic bodies decomposing 



carbonic acid. 

 In general symbols, from the identity 



A=B 



we can infer the identity 



AC = BC. 



This is but a case of plain substitution ; for by the first 

 Law of Thought it must be admitted that 



AC -AC, 



and if in the second side of this identity we substitute 

 for A its equivalent B, we obtain 



In like manner from the partial identity 



A = AB 



we may obtain 



AC = ABC 



by an exactly similar form of substitution ; and in every 



