A long and important fragment of the Περὶ μοψσικῆς of Theophrastus is preserved in Porphyry's commentary on Ptolemy's Harmonics. Both Porphyry and Ptolemy were reedited earlier in this century by Düring, in works which have rightly been taken to supersede the texts of Wallis: and so far as the Theophrastus passage is concerned, we should expect to be able to abandon in Düring's favour the text published by Wimmer, who in effect reprints Wallis, though adopting a few variant readings and emendations from Schneider. But it seems to me that Düring's text is not in all respects an improvement, and that the comments made on it in a subsequent publication by Alexanderson have muddied the waters still further. It is not only a matter of the text: Alexanderson prints also a (partial) translation and an interpretative commentary, and both are open to serious objections. I intend in this paper to deal only with a portion of the fragment, but it is that portion whose argument is the most intricate, and one which ought to shed a good deal of light on central controversies among the musical theoreticians who follow Aristotle. I am not in a position to dispute any of Düring's findings in the manuscripts, but where emendation has in any event proved necessary or where the manuscripts differ among themselves, I hope to show through a study of the content of the argument that the case in favour of Düring is not always closed.