Philosophy
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Modus Tollens is a rule of inference in propositional logic that states that if we have a Material Condtional that has a false consequent, then we may infer the negation of the antecedent. In other words, if P implies Q is true and Q is false, then we may infer that P is false. In rule form, this is often expressed as

Natural Deduction Transformation Rules
Rules of inference

Modus Ponens | Modus Tollens | Disjunctive Syllogism | Hypothetical Syllogism | Conjunction Introduction | Conjunction Elimination | Disjunction Introduction | Disjunction Elimination | Bicondional Introduction | Biconditional Elimination | Constructive Dilemma | Destructive Dilemma | Absorption | Modus ponendo tollens

Rules of Transformation
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