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This process is called Obversion, Permutation or Immediate Inference by Privative Conception; it is applicable to every proposition including O. A further process, known as Contraposition or Conversion by Negation, consists of conversion following on obversion.

Another term, Inversion, has been used by some logicians for a still more complicated process by the alternative use of conversion and obversion, which is applicable to A and E, and results in obtaining a proposition concerning the contradictory of the original subject; thus “all A is B” becomes “some not-A is not B.”

Hence the rule of Obversion;—Substitute for the predicate term its Contrapositive,5 and change the Quality of the proposition.

Dr. Bain includes also Material Obversion, the analogue of Formal Obversion applied to a Subject.

Thus in Obversion and Conversion by Contraposition, the homogeneity of the negative term is tacitly assumed; it is assumed that A and not-A are of the same kind.