Contrapositive

The contrapositive is the logically equivalent flip of a conditional. From "If A, then B," the contrapositive is "If not B, then not A." It says exactly the same thing in reverse, and it is always valid.

Example

"If the alarm sounds, the doors lock" becomes "If the doors did not lock, the alarm did not sound."

See also

• Conditional Statement
• Only If
• Unless

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Last updated October 12, 2026