CHAPTER V. 



DISJUNCTIVE PROPOSITIONS. 



IN the previous chapter I have exhibited various forms 

 of deductive reasoning by the process of substitution, so 

 far as they can be treated without the use of disjunctive 

 propositions ; but we cannot long defer the consideration 

 of this more complex class of identities. General terms 

 arise, as we have seen (p. 29), from classifying or men- 

 tally uniting together all objects which agree in certain 

 qualities, the value of this union consisting in the fact 

 that the power of knowledge is multiplied thereby. In 

 forming such classes or general notions, we overlook or 

 abstract the points of difference which exist between the 

 objects joined together, and fix our attention only on the 

 points of agreement. But every process of thought may 

 be said to have its inverse process, which consists in 

 undoing the effects of the direct process. Just as division 

 undoes multiplication, and evolution undoes involution, 

 so we must have a process which undoes abstraction, or 

 the operation of forming general notions. This inverse 

 process will consist in distinguishing the separate objects 

 or minor classes which are the constituent parts of any 

 wider class. When we mentally unite together certain 

 objects visible in the sky and call them planets, we shall 

 afterwards need to distinguish the contents of this general 

 notion, which we do in the disjunctive proposition 



A planet is either Mercury or Venus or the Earth or 



or Neptune. 



Having formed the very wide class 'vertebrate animal/ 



