obversion

American  

[ob-vur-zhuhn, -shuhn] / ɒbˈvɜr ʒən, -ʃən /

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.”

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Etymology

Origin of obversion

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

Example Sentences

Similarly, if it be false that No men are wise, which by obversion is equivalent to All men are not-wise, then it is true at least that Some men are wise.

From Logic Deductive and Inductive by Read, Carveth

Similarly, No men are wise, being by obversion equivalent to All men are not-wise, is incompatible with Some men are wise, by the same principle of Contradiction.

From Logic Deductive and Inductive by Read, Carveth

Contraposition needs no justification by the Laws of Thought, as it is nothing but a compounding of conversion with obversion, both of which processes have already been justified.

From Logic Deductive and Inductive by Read, Carveth

Of 'Some men,' therefore, by the principle of Excluded Middle, 'not-wise' is to be affirmed; and Some men are not-wise, is by obversion equivalent to Some men are not wise.

From Logic Deductive and Inductive by Read, Carveth

Hence, since The sky is blue becomes by obversion, The sky is not not-blue, we may also infer The sky is not red, etc.

From Logic Deductive and Inductive by Read, Carveth