Constructive dilemma is a name of a valid rule of inference of propositional logic. It is the inference that, if P implies Q and R implies S and either P or R is true, then Q or S has to be true. In sum, if two conditionals are true and at least one of their antecedents is, then at least one of their consequents must be too. Constructive dilemma is the disjunctive version of modus ponens, whereas,destructive dilemma is the disjunctive version of modus tollens. The rule can be stated: where the rule is that whenever instances of " ", " ", and " " appear on lines of a proof, "