Thus Plotinus arrives at practically what was to be the usual Christian position in the middle ages regarding the influence of the stars, maintaining the freedom of the human will and yet allowing a large field to astrological prediction. He is evidently more concerned to combat the notion that the stars cause evil or are to be feared as evil powers than he is to combat the belief in their influence and significations. His speaking of the stars both as signs and causes in a way doubles the possibility of prediction from them. If he attacked the language used by astrologers of the planets, and perhaps to a certain extent the technique of their art, he supported astrology by reconciling the existence of evil and of human freedom with a great influence of the stars and by his emphasis upon the importance of the figures made by the movements of the heavenly bodies above any purely physical effects of their bodies as such. Thus he reinforced the conception of occult virtue, always one of the chief pillars, if not the chief support, of occult science and magic. On the other hand, men were not likely to reform a language and technique sanctioned by as great an astronomer as Ptolemy merely because a Neo-Platonist questioned its propriety. [Sidenote: Porphyry’s _Letter to Anebo_.] Although Plotinus denied that diseases were due to demons, we once heard him speak of “demonic nature,” and one of the _Enneads_ discusses _Each man’s own demon_. Here, however, the discussion is limited to the power presiding in each human soul, and nothing is said of magic. For the connection of demons with magic and for the art of theurgy we must turn to the writings of Porphyry and Iamblichus, and especially to _The Letter to Anebo_ of Porphyry, who lived from about 233 to 305, and the reply thereto of the master Abammon, a work which is otherwise known as _Liber de mysteriis_[1365]. The attribution of the latter work to Iamblichus, who died about 330, is based upon an anonymous assertion prefixed to an ancient manuscript of Proclus and upon the fact that Proclus himself quotes a passage from the _De mysteriis_ as the words of Iamblichus. This attribution has been questioned, but if not by Iamblichus, the work seems to be at least by some disciple of his with similar views[1366]. Other works of Iamblichus are largely philosophical and mathematical; among the chief works of Porphyry, apart from his literary work in connection with Plotinus, were his commentaries on Aristotle and fifteen books against the Christians. [Sidenote: Its main argument.]