Numenius has been called both a pythagorizing Platonist and a platonizing Pythagorean. However, the key to his attitude toward philosophy is summed up in his own statement that "Plato pythagorizes" (P. Henry 1991, p. lxx). He took the mysterious passage about the three kings in the Platonic Second Letter as coming from Socrates, and he likely used this passage as support for the triad of gods which he posited as first principles. Plato and Pythagoras were considered by him as the twin sources of philosophical truth, with which the traditions of the Hebrews, Egyptians, the Zoroastrian Magi, and even the Brahmins were all in agreement.
Numenius' triad of gods begins with the First God, called also the Good, who is eternal, immutable, and at rest, concerned only with the intellectual realm. He is likened by Numenius to the owner of a farm who, after having sown the fields, leaves it up to his farmhands to cultivate the crops. The Second God, called Mind and Demiurge is responsible for translating the things of the intellectual realm to the realm of matter, thereby establishing a cosmos. In this capacity the Second God is called World-Soul. However, once this Soul comes into contact with matter, the source of all evil according to Numenius, it becomes divided into a rational and an irrational part, the former remaining in contemplation of the divine realm, and the latter immersing itself in the material realm. It is not clear whether Numenius intended to posit two World-Souls (one good, one evil) or if he had in mind simply a division within that Soul of an irrational and a rational part. If Numenius' triad involves a strict separation of three distinct divinities (and this is a matter of interpretation) then we should speak of a separate World-Soul that is evil. If the triad is intended to imply a three-fold series of activities emanating from the divine realm, then we are correct in assuming that Numenius posited a single World-Soul with two warring parts. Due to the fragmentary nature of his surviving writings, however, it is impossible to know for sure what he intended.
Human souls were described by Numenius as divine fragments of the Demiurge, each one a microcosm of both the intellectual and the physical realm (Tripolitis, pp. 26-30). He taught that all souls contain both a rational and an irrational element, the former derived from the Second God, the latter from association with the material realm. Numenius taught that souls enter the cosmos by way of the Tropic of Cancer, acquiring various characteristics as they pass through the seven planetary spheres. The soul that leads a virtuous life – which for Numenius meant living a contemplative life detached from bodily things – will re-ascend to heaven (the sphere of the fixed stars) by way of the Tropic of Capricorn. The soul that fails to lead a correct life will enter Hades (located by Numenius in the mists above the world) where it will undergo chastisement until reincarnated in another body suitable to its nature. Numenius taught that certain souls may become so corrupted that they will enter the bodies of animals. In a doctrine that likely influenced Origen (in his doctrine of multiple ages), Numenius taught that the series of reincarnations are finite, and will eventually lead the soul back to the divine realm, though how this is accomplished for a soul existing in animal bodies is not entirely clear, since such a soul is presumably not susceptible to any rational exhortations to virtue.
No overtly ethical fragments of Numenius' works survive, but we do know that he considered existence in this realm a struggle, with the irrational part of the soul in constant strife with the rational. Salvation from this state only takes place when the soul leaves the material realm for the divine. One is reminded of St. Paul's lament in Romans 7:18-23 where he describes the war taking place between his flesh (body, matter) and his mind. His mind knows the good, he says, but his flesh continually prevents him from achieving this good. It is possible that Numenius read St. Paul, but more likely that the two thinkers simply were responding to a shared intellectual milieu consisting not only of Platonic philosophy, but Gnostic and Hermetic doctrines as well.
The influence of Numenius extended well beyond his life-time; his doctrines are recorded in the writings of later Neoplatonists like Porphyry and Proclus, and Plotinus himself was at one point accused of plagiarizing Numenius (Porphyry, Life of Plotinus 17). In the case of Plotinus, we see a clear Numenian influence regarding the triadic arrangement of principles, although Plotinus developed this basic notion in a quite original way. Plotinus also responded to Numenius' doctrine of an evil World-Soul, developing in the process a quite sophisticated doctrine concerning matter and the nature of evil.