Introducing his commentary on Ptol. Harm. I.5, Porphyry discusses at length the vocabulary of musical intervals and in particular the question whether the words λόγος and διάστηµα should or not be used as synonyms. This paper aims, on the one hand, at analysing the way in which he chooses and arranges his sources; on the other hand, at restoring them to the original debate to which they belong—a debate in which a seminal role seems to have been played by Plato’s Timaeus and the contributions of its early commentators (Eratosthenes, Aelianus, Panaetius) in the framework of post-Aristoxenian harmonics. [http://booksandjournals.brillonline.com/content/10.1163/22129758-12341245;jsessionid=410e6ovugpbcq.x-brill-live-01]