A syllogism consists of two statements, the major and the minor premiss, with the conclusion that can be drawn from them. Thus, to take an instance, [fig. 49]. It is evident, from looking at the successive figures that, if we know that the region M lies altogether within the region P, and also know that the region S lies altogether within the region M, we can conclude that the region S lies altogether within the region P. M is P, major premiss; S is M, minor premiss; S is P, conclusion. Given the first two data we must conclude that S lies in P. The conclusion S is P involves two terms, S and P, which are respectively called the subject and the predicate, the letters S and P being chosen with reference to the parts the notions they designate play in the conclusion. S is the subject of the conclusion, P is the predicate of the conclusion. The major premiss we take to be, that which does not involve S, and here we always write it first.
There are several varieties of statement possessing different degrees of universality and manners of assertiveness. These different forms of statement are called the moods.
We will take the major premiss as one variable, as a thing capable of different modifications of the same kind, the minor premiss as another, and the different moods we will consider as defining the variations which these variables undergo.
There are four moods:—
1. The universal affirmative; all M is P, called mood A.
2. The universal negative; no M is P, mood E.
3. The particular affirmative; some M is P, mood I.
4. The particular negative; some M is not P, mood O.
Figure 50.