Olympiodorus led the Platonist school of philosophy at Alexandria for several decades in the sixth century, and both Platonic and Aristotelian commentaries ascribed to him survive. During this time the school’s attitude to the teaching of Aristotelian syllogistic, originally owing something to Ammonius, changed markedly, with an early tendency to reinforce the teaching of syllogistic even in Platonist lectures giving way to a greater awareness of its limitations. The vocabulary for arguments and their construction becomes far commoner than the language of syllogistic and syllogistic figures, and also of demonstration. I discuss the value of these changes for the dating of certain works, especially where the text lectured on does not demand different emphases. The commitment to argument rather than to authority continues, but a greater emphasis eventually falls on the establishment of the premises than on formal validity.