Dear Stephen,
The general concensus in Josephus studies is that Josephus never used Posidonius directly; the one time Posidonius is mentioned (Contra Apionem 2.79) Josephus is thought to have drawn on a secondary source. But it is possible to entertain the notion that Ant. 13 derives from Strabo (and of course Strabo used Posidonius among many other sources, so if one could show Ant. 13 derived from Strabo, one could in turn argue that Strabo drew on Posidonius there). I agree with Shutt et al that Strabo is not always cited when used by Josephus. Further, I'm quite familiar with Katherine Clarke's, Between History and Geography, although she doesn't really say much new, Felix Jacoby having masterfully argued the overlap between hellenistic history and geography. I don't know how well-read you are in the Classics, but historians frequently wrote an excursus on geography and ethnography before dealing with the history of a region (this practice going back at least to Herodotus).
What is at issue here (with reference to Ant. 13) is that these topics are virtually always introduced as informative background to a historical episode. To cite some examples which may be familiar to you, Josephus later in Ant. 13 discusses the Pharisees and Sadducees as background of the defection of John Hyrkanus to the latter; the three Jewish sects are discussed in War 2 to shed light on the revolt of Judah the Galilean; both Diodorus Siculus (following Theophanes of Mytilene) and Tacitus inserted an ethnographical excursus on the Jews when about to tell a story of Jerusalem's fall. In Ant. 13 the brief description of the three sects has no connection to the surrounding material: it is not attached to a historical episode as one would expect if taken from a historian's work. Nor does the hypothesis that an associated historical episode in the time of Jonathan (presumably involving one of the Jewish sects) was simply omitted, since Josephus was eager to incorporate historical references to the Jews found in Graeco-Roman sources. Additionally, Ant. 13 is basically a paraphrase of Maccabees until the latter source "ran out" in the time of John Hyrkanus, when Josephus turned to Strabo, Timagenes, Nicolas of Damascus, and other such writers. For all these reasons, it seems doubtful that Josephus found the reference to the three Jewish sects in a historical account dealing with events in the 140s BCE. It seems to me the lack of historical context indicates the passage came from a purely ethnographical essay. In any case, Strabo's Geography, when discussing the Jews (in a lengthy passage most view as dependent on Posidonius) shows no trace of knowledge of Jewish sects.
In the past, if I recall correctly, you have given credibility to the three sects having existed in the time of Jonathan based on a hypothesis that the passage in Ant. 13 was based on actual historical data known to Strabo or Posidonius. And in turn, the passage occurring during Josephus' account of the events of the 140s BCE gave credibility to a Posidonian authorship, his history beginning about 146 BCE. Now you are saying this account of the Jewish sects did not appear at the outset of his (or Strabo's) history, chronologically speaking. Do you still believe one of these authors related an episode in Jonathan's time and attached the excursus under question? If so, how do you account for the absence of that episode in Josephus?
Best regards,
Russell Gmirkin
