Statements and fragments of his apparently very numerous works have been preserved by Origen, Theodoret, and especially by Eusebius, and from them we may learn the nature of his Platonist-Pythagorean philosophy, and its approximation to the doctrines of Plato.
Numenius was a Neopythagorean, but his object was to trace the doctrines of Plato up to Pythagoras, and at the same time to show that they were not at variance with the dogmas and mysteries of the Brahmins, Jews, Magi and Egyptians. His intention was to restore the philosophy of Plato, the genuine Pythagorean and mediator between Socrates and Pythagoras in its original purity, cleared from the Aristotelian and Stoic doctrines, and purified from the unsatisfactory and perverse explanations, which he said were found even in Speusippus and Xenocrates, and which, through the influence of Arcesilaus and Carneades had led to a bottomless skepticism. His work on the apostasy of the Academy from Plato, to judge from its rather numerous fragments, contained a minute and wearisome account of the outward circumstances of those men, and was full of fabulous tales about their lives, without entering into the nature of their skepticism.
George Karamanolis from the Stanford Encyclopedia of Philosophy noted, "The remains of Numenius' work leave no doubt that he relied primarily on texts of Plato in constructing his own system of principles. Ancient testimonies are, however, divided between those that classify him as a Platonist philosopher (Porphyry, Life of Plot. 14.12, Eusebius, Prep. Ev. XI.21.7) and those that consider him a Pythagorean (Origen, Against Celsus I.15, VI.51, V.38 frs. 1b–1c, 53, Porphyry, Ad Gaurum 34.20–35.2; fr. 36, Calcidius, In Timaeum 297.8 Waszink; fr. 52.2). We should not see any contradiction or even tension in this double classification. Numenius is a Pythagorean Platonist like Moderatus half a century earlier or Eudorus around the turn of the millenium. That is, Numenius accepted both Pythagoras and Plato as the two authorities one should follow in philosophy, but he regarded Plato's authority as subordinate to that of Pythagoras, whom he considered to be the source of all true philosophy—including Plato's own. For Numenius it is just that Plato wrote so many philosophical works, whereas Pythagoras' views were originally passed on only orally (cf. fr. 24.57-60)."
His books On the Good (Peri Tagathou - Περὶ Τἀγαθοῦ) seem to have been of a better kind; in them he had minutely explained, mainly in opposition to the Stoics, that existence could neither be found in the elements because they were in a perpetual state of change and transition, nor in matter because it is vague, inconstant, lifeless, and in itself not an object of our knowledge; and that, on the contrary, existence, in order to resist the annihilation and decay of matter, must itself rather be incorporeal and removed from all mutability, in eternal presence, without being subject to the variation of time, simple and imperturbable in its nature by its own will as well as by influence from without. True existence is identical with the first god existing in and by itself, that is, with good, and is defined as spirit (nous). But as the first (absolute) god existing in itself and being undisturbed in its motion, could not be creative (demiurgikos - δημιουργικός), he thought that we must assume a second god, who keeps matter together, directs its energy to it and to intelligible essences, and imparts its spirit to all creatures; its mind is directed to the first god, in whom it beholds the ideas according to which it arranges the world harmoniously, being seized with a desire to create the world. The first god communicates its ideas to the second, without losing them itself, just as we communicate knowledge to one another, without depriving ourselves of it. In regard to the relation existing between the third and second god, and to the manner in which they also are to be conceived as one (probably in opposition to the vague duration of matter), no information can be derived from the fragments which have come down to us.