Thus, if _No S is not-P_ be given us to recover the obvertend or to find the corresponding affirmative; the proposition being formally negative, we apply the rule for obverting negatives: 'Remove the negative sign, and for the predicate substitute its contradictory.' This yields the affirmative _All S is P_.

Similarly, to obtain the obvertend of _All S is not-P_, apply the rule for obverting Affirmatives; and this yields _No S is P_.
Sec.6.

Contrariety .-- We have seen in chap.iv.Sec.8, that contrary terms are such that no two of them are predicable in the same way of the same subject, whilst perhaps neither may be predicable of it.

Similarly, Contrary Propositions may be defined as those of which no two are ever both true together, whilst perhaps neither may be true; or, in other words, both may be false.

This is the relation between A.and E.when concerned with the same matter: as A .-- _All men are wise_; E .-- _No men are wise_.