contrapositive

In logic, the contrapositive of a conditional statement is found by inverting both the antecedent and consequent and then switching their order in the relationship. For example, the contrapositive of pq is ¬q→¬p.

A conditional statement and its contrapositive are logically equivalent. This is evident by viewing their truth table but can also be seen through example. Consider the statement all iguanas are reptiles, i→r.  The contrapositive is ¬r→¬i ,or if something is not a reptile then it is not an iguana.

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