306 THE SCIENCE OF LOGIC 



to belong to the class [M] : not therefore of anything more definite, 

 of anything more, than this 5 [of the minor premiss]. Hence if 

 the 5 be undistributed in the minor it must remain undistributed 

 in the conclusion. Similarly, the Dictum requires that the pre 

 dication be made in the same way in the conclusion as in the 

 major premiss. If, therefore, the predication is made here by 

 means of an undistributed term \P\ this term must remain un 

 distributed in the conclusion. Hence the rule : No term may 

 be distributed in the conclusion which was undistributed in its 

 premiss. 



(5) The Dictum provides that the minor premiss be affirma 

 tive, for it asserts that something belongs to a certain class. 

 Generalizing this, so as to make it applicable to other forms of 

 syllogism, we have the rule : One at least of the premisses of a syl 

 logism must be affirmative. 



(6) The Dictum recognizes the possibility of the original 

 predication being negative, but demands that in such a case the 

 predication in the conclusion be also negative, and that if the pre 

 dication in the major (as well as in the minor) be affirmative the 

 predication in the conclusion must also be affirmative : what 

 is predicated in the major must always be predicated similarly, 

 in like manner, in the conclusion. Generalizing this we have the 

 rule : A negative premiss necessitates a negative conclusion, and 

 vice versa. 



Summing up these results we have the following six GENERAL 

 RULES OR CANONS OF SYLLOGISM. 



A. Rules of Structure : 



(1) A SYLLOGISM MUST CONTAIN THREE, AND ONLY 

 THREE, TERMS ; 



(2) A SYLLOGISM MUST CONTAIN THREE, AND ONLY THREE, 

 PROPOSITIONS. 



B. Rules of Quantity : 



(3) THE MIDDLE TERM MUST BE DISTRIBUTED AT LEAST 

 ONCE IN THE PREMISSES ; 



(4) NO TERM MAY BE DISTRIBUTED IN THE CONCLUSION 

 WHICH WAS NOT DISTRIBUTED IN ITS PREMISS. 



C. Rules of Quality : 



(5) ONE AT LEAST OF THE PREMISSES MUST BE AFFIRMA 

 TIVE ; 



(6) A NEGATIVE PREMISS NECESSITATES A NEGATIVE CON 

 CLUSION, AND VICE VERSA. 



