A logical principle stating that a conditional statement and its contrapositive are logically equivalent and therefore always have the same truth value.
From Latin 'contrapositus' + '-ion' suffix. The term crystallized in medieval logic as scholars formalized the rules of valid reasoning inherited from Aristotle.
This principle is so fundamental that it's used in everything from legal argumentation to debugging computer code—whenever you need to verify something by proving its negative form works!
Complete word intelligence in one call. Free tier — 50 lookups/day.