In explicating a passage from Physics A 3, Simplicius reports a criticism by Alexander of Aphrodisias against Plato’s conception of not-being in the Sophist. Alexander deems this conception contradictory, because it posits that unqualified not-being is. Simplicius defends Plato and gives a diagnosis of what he regards as Alexander’s interpretative mistake in raising his objection. I unpack this debate and bring out ways in which it sheds light on important aspects of Plato’s project in the Sophist and of Simplicius’ own (...) philosophical background, notably in Damascius’ De principiis. (shrink)