Hermes Trismegistus (“Hermes Thrice Great”), the legendary founder of the Hermetic tradition and the inventor of astrology, alchemy, medicine, and magic, was literally a figure to conjure with from the dawn of the Middle Ages until the seventeenth century. In Greek, Hermes is the messenger of the gods. In Latin, he is known as Mercurius, and Mercury, the volatile, changeable matter, plays a