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modus tollens

noun
modus tol·​lens | \-ˈtäˌlenz\
plural modi tollentes\-​(ˌ)täˈlen‧ˌtēz \

Definition of modus tollens 

: a mode of reasoning from a hypothetical proposition according to which if the consequent be denied the antecedent is denied (as, if A is true, B is true; but B is false; therefore A is false)

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