Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Logic A proposition derived by negating and permuting the terms of another, equivalent proposition; for example, All notY is notX is the contrapositive of All X is Y.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. the inverse of the converse of a given proposition
from The Century Dictionary and Cyclopedia
 Of, pertaining to, or produced by contraposition: as, contrapositive propositions; contrapositive conversion.
 n. In logic, a contrapositive proposition.
Etymologies
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Examples

I'm leaving out some steps here (which students always appropriately resent), but the above is all of a piece with something called the "contrapositive".

The '' 'contrapositive' '' of a statement of the form "A implies B" is the [[logic]] ally equivalent statement "notB implies notA."

We can also see this in the contrapositive: having more than one sexual partner is by definition nonmonogamous.

A natural experiment that is the contrapositive to Korea would be Vietnam.
The Ratios, Bryan Caplan  EconLog  Library of Economics and Liberty

The contrapositive of this is that if you are not a U.S. national (e.g. you lose your nationality through the operation of the law) then it follows you are also not a citizen.
The Volokh Conspiracy » Peter Spiro on Lieberman’s CitizenshipStripping Legislation

(If Messiahs are in the habit of denying themselves, surely the contrapositive is true.)

But mythological creatures also performed a second, often contrapositive, argumentative function; commentators who rejected evolution regularly did so by dismissing these creatures.

If Messiahs are in the habit of denying themselves, surely the contrapositive is true.

The contrapositive of this is that if you are not a U.S. national e.g. you lose your nationality through the operation of the law then it follows you are also not a citizen.
The Volokh Conspiracy » Peter Spiro on Lieberman’s CitizenshipStripping Legislation

Why not look as well to where such contrapositive energy has always been found in literature: as a function of literary representing per se, the writing itself?
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