320 THE SCIENCE OF LOGIC 



2. M is predicate of both premisses in the SECOND FIGURE ; 



3. M is subject of both premisses in the THIRD FIGURE ; 



4. M is predicate of the major and subject of the minor pre 

 miss in the FOURTH FIGURE. 



1 60. DISTINCTION OF MOODS. By the Mood or Mode 

 (&quot; Modus &quot;) of a syllogism, we understand the quality and quantity 

 of its premisses and conclusion. Distinctions tfmood result, there 

 fore, from a consideration of the quality and quantity of the pro 

 positions of the syllogism its proximate matter. For example, 

 AAA expresses a syllogism in which major, minor, and conclusion 

 are universal affirmatives ; E I O expresses a syllogism in which 

 the major is a universal negative, the minor a particular affirmative \ 

 and the conclusion a particular negative. 



Now, recognizing the distinction between major and minor 

 premisses (148), and always writing the major first, we can easily 

 understand that there are, a priori, sixteen possible arrangements 

 of the propositions A, E, I, O, taken two at a time, as premisses 

 of a syllogism. These are A A, A I, A E, A O, I A, (I I), (I E), 

 (I O), E A, E I, (E E), (E O), O A, (O I), (O E), (OO). Further 

 more, each of these sixteen combinations of premisses may be ex 

 pressed in four different figures. For example, A A may represent 

 (i) M aP, S aM ; (2) P a M, S a M; (3) M a /&amp;gt;, Ma S; 

 (4) P a M , Ma 5. Thus, we have in all sixty-four possible forms of 

 syllogism ultimately distinct from one another when determined 

 by figure and mood. The question now, therefore, arises 1 : How 

 many of these possible moods are valid? How many of them yield 

 conclusions ? This we shall proceed to determine by examining 

 the sixteen forms given above, in the light of the General Rules and 

 Corollaries (154-6), and by putting them into each of the four 

 figures successively. The latter process will simultaneously bring 

 to light certain rules which apply to the separate figures of the 

 syllogism, and which are known as the Special Rules of Figure : 

 to distinguish them from the general rules of syllogism (154), of 

 which they are simply narrower applications. 



161. DETERMINATION OF LEGITIMATE MOODS AND SPECIAL 

 RULES OF EACH FIGURE. If we apply the general rules and 

 corollaries to the sixteen combinations given above, we shall find 

 that E E, E O, O E and O O violate Rule 5, which forbids two neg 

 ative premisses ; that I I, I O, and O I are excluded by the first 

 corollary, which forbids two particulars ; and that I E is excluded 

 by the third corollary, which forbids the combination of a particu- 



