DISJUNCTIVE JUDGMENTS AND PROPOSITIONS 291 



must take the assertoric form which gives alternative predi 

 cates. Care must be taken, in obversion, to secure the correct 

 negative or contradictory of the original alternative predicate. 

 The required form is secured by substituting for the simple terms 

 their contradictories, for alternative combinations conjunctive 

 ones, and for conjunctive combinations alternative ones. 1 Thus, 

 from the A proposition we may derive the following eductions : 

 Original Proposition Every S is either P or Q. 

 Obverse . . . No S is both P and Q. 

 Converse . . . Some things that are either P or Q are S. 

 Obverted Converse . S ome things that are either P or Q are not S. 

 Partial Contrapositive Nothing that is both P and Q is S. 

 Full Contrapositive . Everything that is both P and Q is S. 

 Partial Inverse . . Some S s are neither P nor Q. 

 Full Inverse . . Some S s are both P and Q. 



From the I proposition we may infer as follows : 

 Original Proposition Some S s are either P or Q. 

 Obverse . . . Some S s are not both P and Q. 

 Converse . . Some things that are either P or Q are S. 



Obverted Converse . S ome things that are either P or Q are not S. 



Were we to take the alternative form which gives us a choice 

 between two independent judgments &quot;Either X is true or Y 

 is true &quot; we might set down as a form analogous to the obverse 

 the disjunctive form &quot; X and Y are not both true &quot;. But, since these 

 forms have no elements analogous to subject and predicate in the 

 categorical, we cannot derive any of the ordinary eductions 

 from them. 



KEYNES, Formal Logic, pt. ii., chap. x. ; Appendix C., chap. i. WELTON, 

 Logic, i., pp. 187-92, 246, 274. JOSEPH, Logic, pp. 167-8. VENN, Em 

 pirical Logic, pp. 242-48. 



1 KEYNES, op. cit,, p. 488. 



