In the newly-discovered “On Principles and Matter”—we can definitely ascertain by Porphyry—the author concludes that there must be two principles responsible for all beings, or at least all sensible beings: God (the active cause) and matter (the passive cause). In large part this agrees with Atticus’ position, which the text also quotes, and which we also know Porphyry engaged with vigorously, from Proclus’ Timaeus Commentary. However there is a something odd about this text’s Porphyry: we seem to have a positive (...) argument for much of Antiochus’ framework of two principles, while in Proclus’ testimony we have a negative argument for the very same position. In this paper I analyze in detail the positive argument for the two principles given in “On Principles”, and I consider the degree to which Porphyry may be supplying a dialectical argument at this point in the text, before eventually criticizing the same view elsewhere (as in Proclus’ testimony). I also consider other points of contact for the text’s two-principles framework with Porphyry’s other extant works and show some consequences for Porphyry’s overall position. (shrink)