Iamblichus (Gr.). A great Theurgist, mystic, and writer of the third and fourth centuries, a Neo-Platonist and philosopher, born at Chalcis in Cœle-Syria. Correct biographies of him have never existed because of the hatred of the Christians; but that which has been gathered of his life in isolated fragments from works by impartial pagan and independent writers shows how excellent and holy was his moral character, and how great his learning. He may be called the founder of theurgic magic among the Neo-Platonists and the reviver of the practical mysteries outside of temple or fane. His school was at first distinct from that of Plotinus and Porphyry, who were strongly against ceremonial magic and practical theurgy as dangerous, though later he convinced Porphyry of its. advisability on some occasions, and both master and pupil firmly believed in theurgy and magic, of which the former is principally the highest and most efficient mode of communication with one’s Higher Ego, through the medium of one’s astral body. Theurgic is benevolent magic, and it becomes goetic, or dark and evil, only when it is used for necromancy or selfish purposes; but such dark magic has never been practised by any theurgist or philosopher, whose name has descended to us unspotted by any evil deed. So much was Porphyry (who became the teacher of Iamblichus in Neo-Platonic philosophy) convinced of this, that though he himself never practised theurgy, yet he gave instructions for the acquirement of this sacred science. Thus he says in one of his writings, “Whosoever is acquainted with the nature of divinely luminous appearances fasmata ( knows also on what account it is requisite to abstain from all birds (and animal food) and especially for him who hastens to be liberated from terrestrial concerns and to be established with the celestial gods”. (See Select Works by T. Taylor, p. 159.) Moreover, the same Porphyry mentions in his Life of Plotinus a priest of Egypt, who, “at the request of a certain friend of Plotinus, exhibited to him, in the temple of Isis at Rome, the familiar daimon of that philosopher “. In other words, he produced the theurgic invocation (see “Theurgist”) by which Egyptian Hierophant or Indian Mahâtma, of old, could clothe their own or any other person’s astral double with the appearance of its Higher EGO, or what Bulwer Lytton terms the “ Luminous Self”, the Augoeides, and confabulate with It. This it is which Iamblichus and many others, including the mediæval Rosicrucans, meant by union with Deity. Iamblichus wrote many books but only a few of his works are extant, such as his “Egyptian Mysteries” and a treatise “On Dæmons”, in which he speaks very severely against any intercourse with them. He was a biographer of Pythagoras and deeply versed in the system of the latter, and was also learned in the Chaldean Mysteries. He taught that the One, or universal MONAD, was the principle of all unity as well as diversity, or of Homogeneity and Heterogeneity; that the Duad, or two (“ Principles”), was the intellect, or that which we call Buddhi-Manas; three, was the Soul (the lower Manas), etc. etc. There is much of the theosophical in his teachings, and his works on the various kinds of dæmons (Elementals) are a well of esoteric knowledge for the student. His austerities, purity of life and earnestness were great. Iamblichus is credited with having been once levitated ten cubits high from the ground, as are some of the modern Yogis, and even great mediums.
Source: H.P.Blavatsky - The Theosophical Glossary
Iamblichus. A great Theosophist and an Initiate of the third century. He wrote a great deal about the various kinds of demons who appear through evocation, but spoke severely against such phenomena. His austerities, purity of life and earnestness were great. He is credited with having been levitated ten cubits high from the ground, as are some modern Yogis, and mediums.
Source: H.P.Blavatsky - The Key to Theosophy, The theosophical glossary