obversion

[ ob-vur-zhuhn, -shuhn ]

noun

1. an act or instance of obverting.
2. something that is obverted.
3. Logic. a form of inference in which a negative proposition is obtained from an affirmative, or vice versa, as “None of us is immortal” is obtained by obversion from “All of us are mortal.”

Word History and Origins

Origin of obversion¹

1840–50; < Late Latin obversiōn- (stem of obversiō ) a turning toward, equivalent to obvers ( us ) ( obverse ) + -iōn- -ion

Example Sentences

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.

From Project Gutenberg

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.”

From Project Gutenberg

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

From Project Gutenberg

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

From Project Gutenberg

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.

From Project Gutenberg