Proofs by Reductio ad Impossibile in Aristotle’s Prior Analytics

This paper aims at clarifying the procedure of proofs by reductio ad impossibile in Aristotle’s Prior Analytics, especially elucidating what can be taken as impossibility in such proofs. Traditional interpretation has it that the impossibility in Aristotle’s reductio proofs must be a contradiction. I argue for an alternative interpretation according to which both contrarieties and contradictions are suitable as the impossibility required by the proofs in question. I also present a definition of proof by reductio ad impossibile in accordance with the alternative interpretation.