Hermes Trismegistus — "Thrice-Great Hermes" — is the legendary author of the Hermetic writings: a syncretic figure combining the Greek messenger god Hermes (patron of communication, alchemy, magic and the psychopomp who guides souls to the underworld) with the Egyptian god Thoth (patron of writing, wisdom, magic and the keeper of divine knowledge).
Renaissance scholars believed the Hermetic texts to be more ancient than the Old Testament — perhaps the oldest wisdom in the world, preceding Moses and Plato. When Isaac Casaubon demonstrated in 1614 that the texts were actually composed in the early centuries CE rather than ancient Egypt, it was a significant blow to Hermetic authority — but one that did not end its influence. The ideas proved potent regardless of their age.
The most famous Hermetic text is the Emerald Tablet (Tabula Smaragdina) — a brief, cryptic document of unknown origin whose opening line has become the most quoted axiom in all of Western esotericism: "As above, so below; as below, so above."