obversion

[ ob-vur-zhuhn, -shuhn ]

noun

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.”

Origin of obversion

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

Dr. Bain calls this Material obversion because it cannot be practised safely without reference to the matter of the proposition.
Logic, Inductive and Deductive | William Minto
In this it differs from obversion, Conversion, and Contraposition, each of which stands for one process.
Logic | Carveth Read
Define obversion and Inversion, and apply these processes also to the above three propositions.
Logic | Carveth Read
A further process, known as Contraposition or Conversion by Negation, consists of conversion following on obversion.
Encyclopaedia Britannica, 11th Edition, Volume 7, Slice 3 | Various

Browse