Abstract
In this paper I compare Philoponus’s account of the laws of conversion for categorical and modal propositions with Alexander’s exposition of the same topic. I argue that Philoponus’s main source was Alexander’s commentary on Aristotle’s Prior Analytics and that Philoponus had no access to independent sources to reconstruct Theophrastus’s proof for the conversion of universal negative propositions. I suggest that the different solutions that Alexander and Philoponus offer to the puzzles of the doctrine of the laws of conversion depend on the two commentators’ different exegetical strategies. Alexander tries to solve the puzzles by means of doctrines, which Aristotle expounded elsewhere. Philoponus instead interprets Aristotle’s passage as implying a hierarchy among propositions - a doctrine which is not explicitly present in Aristotle’s text.
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