We have learned that to be logical all categorical assertions must be reduced to some one of the four propositions, A, E, I, O. If these four logical propositions be given the same subject and predicate, certain definite relations will become evident; therefore, Opposition is said to exist between propositions which are given the same subject and predicate, but differ in quality, or in quantity, or in both.
The following illustrative outline will make this clear:
1.
Original Proposition.
I. All men are mortal. (A)
II. No men are immortal. (E)
III. Some men are wise. (I)
IV. Some men are mortal. (I)
V. Some men are not wise. (O)
VI. Some men are not immortal. (O)