Modus Tollens
Modus tollens is a valid argument form in propositional calculus in which p and q are propositions. If p implies q, and q is false, then p is false. Also known as an indirect proof or a proof by contrapositive. (p⇒q, ¬q)/(∴¬p) For example, if being the king implies having a crown, not having a crown implies not being the king.