230 THE SCIENCE OF LOGIC. 



any proposition, taken as true, we derive others implied in it, though 

 differing from it in subject or predicate or both. 



Since each of the two terms in a categorical proposition, 5 

 and P, has a conceivable negative, non-S and non-P (usually 

 written S and P\ every categorical proposition suggests four 

 terms, S, P, S, and P. We may, therefore, inquire how many 

 legitimate predications may be made about 5 or 5 in terms of 

 P or P, and, pice versa, how many about P or P in terms of 5 or 

 S. 



Supposing the subject of the original proposition to be S, we 

 may conceive three other derived propositions whose respective 

 subjects would be P, P and 5. And each of these four terms 

 can have either of two alternative predicates : vS and ,S may have 

 each P or P for predicate ; and P and P may have each 5 or 5 

 for predicate. 



Thus, starting with the form 

 (!) S P, we get 



(2) its obverse . . . S P, by a process called Obversion ; 



(3) its converse . . P S, ,, ,, Conversion; 



(4) its obverted converse P S, ,, obverting the converse ; 



(5) \^ contrapositive . P S, ,, a process called Contraposition ; 



(6) its obverted contra- P S, obverting the contrapositive ; 



positive 



(7) its inverse S P, ,, a process called Inversion ; 



(8) its obverted in- S P, ,, obverting the inverse. 



verse 



It will be noted that the converse, contrapositive, and inverse 

 have positive predicates; and the obverted form of each as well 

 as the obverse of the original negative predicates. 



We have, then, to examine four forms of Eduction, viz. Obver 

 sion, Conversion, Contraposition, and Inversion : and it will be found 

 that the two latter depend on, and are only repeated applications 

 of, the two former. 



117. OBVERSION is that process of immediate inference by which 

 we infer from a given proposition another having for its subject the 

 former subject and for its predicate the contradictory of the former 

 predicate. 



The original proposition is called the obvertend, the inferred 

 proposition the obverse. 



