80 THE PRINCIPLES OF SCIENCE. 



we may specify its subordinate classes thus : ' A verte- 

 brate animal is either a mammalian, bird, reptile, or 

 fish.' Nor is there any limit to the number of possible 

 alternatives. ' An exogenous plant is either a ranun- 

 culus, a poppy, a crucifer, a rose, or it belongs to some 

 one of the other seventy natural orders of exogens at 

 present recognised by botanists/ A cathedral church in 

 England must be either that of London, Canterbury, 

 Winchester, Salisbury, Manchester, or of one of about 

 twenty-four cities possessing such churches. And if we 

 were to attempt to specify the meaning of the term 

 ' star/ we should require to enumerate as alternatives, 

 not only the many thousands of stars recorded in cata- 

 logues, but the many millions yet unnamed. 



Whenever we thus distinguish the parts of a general 

 notion we employ a disjunctive proposition, in at least 

 one side of which are several alternatives joined by the 

 so called disjunctive conjunction or, a contracted form of 

 other. There must be some relation between the parts 

 thus connected in one proposition ; we may call it the 

 disjunctive or alternative relation, and we must carefully 

 inquire into its nature and results. This relation is that 

 of doubt and ignorance, giving rise to choice or uncer- 

 tainty. Whenever we classify and abstract we must open 

 the way to such uncertainty. By fixing our attention on 

 certain attributes to the exclusion of others we necessarily 

 leave it doubtful what those other attributes are. The 

 term ' molar tooth ' bears upon the face of it that it is a 

 part of the wider term ' tooth.' But if we meet with the 

 simple term * tooth' there is nothing to indicate whether it 

 is an incisor, a canine, or a molar tooth. This doubt, 

 however, may be resolved by other information, and we 

 have to consider what are the appropriate logical pro- 

 cesses for treating disjunctive propositions in connection 

 with other propositions disjunctive or otherwise. 



