Every proposition in a standard-form syllogism is an A, E, I or O. Reading the three letters gives a short name for the argument's shape.
Key idea
The mood of a syllogism is the letters of its major premise, minor premise and conclusion, in that order: for example AAA, EIO or OAO.
Why it works
Validity depends only on the pattern, never on what the terms are about. Two syllogisms with the same letters in the same positions have the same pattern of all/some and is/is not, so the mood (together with the figure, next lesson) is enough to decide whether an argument is valid. With 4 choices for each of 3 lines there are 4 × 4 × 4 = 64 moods.
Worked example
Worked example: Reorder, then read
Find the mood: "Some potters are knitters. All knitters are gardeners. So some potters are gardeners."
- The conclusion's predicate is gardeners, so the major premise is "All knitters are gardeners": A.
- The minor premise is "Some potters are knitters": I.
- The conclusion "Some potters are gardeners" is I.
Answer: AII
Worked example: Conclusion first
Find the mood: "Some swimmers are not drummers, since no drummers are artists and some swimmers are artists."
- The conclusion comes first here: "Some swimmers are not drummers" (O). Its predicate, drummers, is the major term.
- Major premise: "No drummers are artists" (E). Minor premise: "Some swimmers are artists" (I).
- Standard order: E, I, O.
Answer: EIO
A mood alone does not settle validity. EIO happens to be valid in every figure, but AAA is valid only in figure 1: "All P are M, all S are M, so all S are P" is not. The next lesson adds the figure.
Common mistake
Reading the letters in the order the sentences appear rather than in standard-form order. In the example above, the spoken order gives IAI, a different mood. Check: the first letter must belong to the premise with the major term.