Hermetic writings, works of revelation on occult, theological, and philosophical subjects ascribed to the Egyptian god Thoth (Greek Hermes Trismegistos [Hermes the Thrice-Greatest]), who was believed to be the inventor of writing and the patron of all the arts dependent on writing. The collection, written in Greek and Latin, probably dates from the middle of the 1st to the end of the 3rd century ad. It was written in the form of Platonic dialogues and falls into two main classes: “popular” Hermetism, which deals with astrology and the other occult sciences; and “learned” Hermetism, which is concerned with theology and philosophy. Both seem to have arisen in the complex Greco-Egyptian culture of the Ptolemaic and Roman periods.

From the Renaissance until the end of the 19th century, popular Hermetic literature received little scholarly attention. More recent study, however, has shown that its development preceded that of learned Hermetism and that it reflects ideas and beliefs that were widely held in the early Roman Empire and are therefore significant for the religious and intellectual history of the time.

In the Hellenistic age there was a growing distrust of traditional Greek rationalism and a breaking down of the distinction between science and religion. Hermes-Thoth was but one of the gods and prophets (chiefly Oriental) to whom people turned for a divinely revealed wisdom.

In this period the works ascribed to Hermes Trismegistos were primarily on astrology; to these were later added treatises on medicine, alchemy (Tabula Smaragdina [“Emerald Tablet”], a favourite source for medieval alchemists), and magic. The underlying concept of astrology—that the cosmos constituted a unity and that all parts of it were interdependent—was basic also to the other occult sciences. To make this principle effective in practice (and Hermetic “science” was intensely utilitarian), it was necessary to know the laws of sympathy and antipathy by which the parts of the universe were related. But because these assumed affinities did not, in fact, exist and hence could not be discovered by ordinary scientific methods, recourse had to be made to divine revelation. The aim of Hermetism, like that of Gnosticism (a contemporary religious-philosophical movement), was the deification or rebirth of mortals through the knowledge (gnosis) of the one transcendent God, the world, and humankind.

The theological writings are represented chiefly by the 17 treatises of the Corpus Hermeticum, by extensive fragments in the Anthologion (Anthology) of Stobaeus, and by a Latin translation of the Asclepius, preserved among the works of Apuleius. Though the setting of these is Egyptian, the philosophy is Greek. The Hermetic writings, in fact, present a fusion of Eastern religious elements with Platonic, Stoic, and Neo-Pythagorean philosophies. It is unlikely, however, that there was any well-defined Hermetic community, or “church.”

Hermetism was extensively cultivated by the Arabs, and through them it reached and influenced the West. There are frequent allusions to Hermes Trismegistos in late medieval and in Renaissance literature.