Note Footnote 5: — Chapter III

The inference, such as it is, depends on the Law of Excluded Middle. Either a term P, or its contradictory, not-P, must be true of any given subject, S: hence to affirm P of all or some S, is equivalent to denying not-P of the same: and, similarly, to deny P, is to affirm not-P. Hence the rule of Obversion;—Substitute for the predicate term its Contrapositive,5 and change the Quality of the proposition.