modus tollens in full, modus tollendo tollens (Latin, ‘removing method’), (1) the argument form ‘If A then B; not-B; therefore, not-A’, and arguments of this form (compare fallacy of denying the antecedent); (2) the rule of inference that permits one to infer the negation of the antecedent of a conditional from that conditional and the negation of its consequent. See also COUNTERFAC- TUALS , FORMAL FALLAC. G.F.S.