Diels lettered these 81a (considered by most a fragment) and 81b (considered a testimony). The fragment is preserved in Philodemus' Rhetoric. Philodemus says, "Rhetoric, according to Heraclitus, is the prince of lies." It seems unlikely that Heraclitus attributed the title "prince of lies" to rhetoric. Philodemus seems to be using Heraclitus' phrase but applying it to something else. That is why I render "according to Heraclitus" as "in the words of Heraclitus." The testimony (bottom section) is a scholiast's note to Euripides' Hecuba about the word κοπίδας. This testimony makes a case for the subject being Pythagoras. Then, the fragment of Heraclitus would have read, "Pythagoras is the prince of lies." Though the case is not very strong --as Robinson makes clear when he says, "Philodemus does not in fact name Pythagoras; for this we have to rely upon a very unclear passage of Timaeus (Historicus), itself quoted (how accurately?) by a scholiast on Euripides' Hecuba" (132)-- most editors add [Pythagoras] as the subject of the fragment.
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