  Diogenes Laertius 10.3’s citation of Book 10 of the Collection of the Philosophers is the only explicit mention of a philosophical work of Philodemus among ancient authors. Close correspondence between some of the last part of On Piety and Cicero’s On the Nature of the Gods 1.25–41 may indicate that Cicero used Philodemus’ book as a source, or that the two had a common source, perhaps Zeno of Sidon. Cicero may also have used contemporary Epicurean sources for the views put by Epicurean surrogates in some of his other works: his character ‘Cicero’ mentions his (or Torquatus’) friends Siro and Philodemus as authorities to whom Torquatus could appeal for help against the attack made on Epicurean ethics by ‘Cicero’ (On Moral Ends 2.119). The approach to the liberal arts signaled by Sextus Empiricus in M 1–6 as characteristic of ‘the Epicureans’ is very similar to what we see in Philodemus’ works, as explained above (Section 2.2.5.3.5); Philodemus and/or Zeno probably served as Sextus’ sources for this material.