 We are fortunate to know the titles of some of Numenius' treatises. One was On the Good, which comprised at least 6 books (Eusebius, Preparatio Evangelica XI.18.22). Apparently, this was one of Numenius' most important works and judging from the existing excerpts, it must have been cast in dialogical form (see frs. 3, 4a; cf. Edwards 2010, 118-120). In it Numenius set out to outline his ontology and theology and, more specifically, to investigate the nature of the first principle, which is divine, the cause of everything, and the source of goodness (hence the title On the Good). Numenius is clearly inspired by Plato's accounts of the Form of the Good in Republic VI (508e-509b) and of the one concerning the goodness of the demiurge in the Timaeus (29e-30b), yet, as I will explain below, he did not identify the demiurge with the Form of the Good. Also important must have been Numenius' work On the Indestructibility of the Soul (Origen, Against Celsus V.57; fr. 29), in at least two books. We know of only one certain fragment of this work, but it is probable that many of the testimonies of later Platonists regarding Numenius' views on the soul reflect claims made in it (Dillon 1977, 364) (see below, Psychology). Quite unclear, however, remains the content of Numenius' treatise On Plato's Secret Doctrines (Eusebius, Prep.  Ev. XIII.4.4; fr. 23). In the sole surviving fragment (fr. 23) Numenius discusses Plato's use of the character Euthyphro in the Euthyphro as representative of Athenian popular religion.  Dillon (1977, 364) has speculated that in this work Numenius proposed allegorical interpretations of aspects of Platonic dialogues. This seems plausible since Numenius appears to have indulged in allegorical interpretation of the cultic practices of various nations, taking them to be philosophically significant (fr. 1a; see below, Numenius' Platonism). But given Numenius' attachment to Pythagoreanism (on which see below, Numenius' Platonism) it may also be the case that in this work Numenius presented doctrines that Platonists traditionally associated with Plato, such as the immortality of the soul, as originally Pythagorean (see Dillon 1988, 121).