Of Permutation.
§ 496. Permutation [Footnote: Called by some writers Obversion.] is an immediate inference grounded on a change of quality in a proposition and a change of the predicate into its contradictory-term.
§ 497. In less technical language we may say that permutation is expressing negatively what was expressed affirmatively and vice versâ.
§ 498. Permutation is equally applicable to all the four forms of proposition.
(A) All A is B.
.'. No A is not-B (E).
(E) No A is B.
.'. All A is not-B (A).
(I) Some A is B.
.'. Some A is not not-B (O).
(O) Some A is not B.
.'. Some A is not-B (I).
§ 499, Or, to take concrete examples—
(A) All men are fallible.
.'. No men are not-fallible (E).