Inverse (Logic)

In logic, if S is a statement of the form P implies Q then the inverse of S is a statement of the form (not P) implies (not Q). S and its inverse are not logical equivalents. For example, let S be the true statement "If I am a human, then I am mortal." The inverse of S is the statement "If I am not a human, then I am not mortal," which is not necessarily true. A truth table makes it clear that S and the inverse of S are not logically equivalent:
P Q ¬P ¬Q PQ ¬P→¬Q
T T F F T T
T F F T F T
F T T F T F
F F T T T T
See also: Converse, Contrapositive, Denying the antecedent.

 

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