Is modus tollens a logical fallacy?
Modus tollens is not a logical fallacy; it is a valid approach to deductive reasoning.
However, syllogisms such as modus tollens are often conflated with formal logical fallacies (or non sequitur fallacies).
The two fallacies that are most easily conflated with modus tollens are affirming the consequent and denying the antecedent.
- Affirming the consequent takes the form “If P, then Q. Q. Therefore, P.” This argument is invalid because P might not be the only potential cause of Q.
- Denying the antecedent takes the form “If P, then Q. Not P. Therefore, not Q.” This argument is fallacious because negating P doesn’t prove that Q is impossible.