Hermes Trismegistus (Greek for "Hermes the thrice-greatest", Greek: Ερμης ο Τρισμεγιστος) or Mercurius ter Maximus in Latin, is the syncretism of the Greek god Hermes and the Egyptian Thoth. In Hellenistic Egypt, the god Hermes was given as epithet the Greek name of Thoth. He has also been identified with Enoch. Other similar syncretized gods include Serapis and Hermanubis.

Hermes Trismegistus might also be explained in Euhemerist fashion as a man who was the son of the god, and in the Kabbalistic tradition that was inherited by the Renaissance, it could be imagined that such a personage had been contemporary with Moses, communicating to a line of adepts a parallel wisdom. (Occultist etymology has connected the two, making of Moses a truncated name and positing a full name, Thothmoses. This is presented in the royal hostage thesis below.) A historian, however, would leave such speculation to the history of alchemy and the 19th-century history of occultism.

Origin

Both Thoth and Hermes were gods of writing and of magic in their respective cultures. Thus the Greek god of interpretive communication was combined with the Egyptian god of wisdom as a patron of Antiquity's pseudosciences of astrology and alchemy. In addition, both gods were psychopomps, guiding souls to the afterlife.

The majority of Greeks, and later Romans, did not accept Hermes Trismegistus in the place of Hermes. The two gods remained distinct from one another. Cicero noted several individuals referred to as "Hermes":

The Hermetic literature added to the Egyptian concerns with conjuring spirits and animating statues that inform the oldest texts, Hellenistic writings of Greco-Babylonian astrology and the newly developed practice of alchemy (Fowden 1993: pp65–68). In a parallel tradition, a Hermetic philosophy rationalized and systematized religious cult practices and offered the adept a method of personal ascension from the constraints of physical being, which has led to confusion of Hermetism with Gnosticism, which was developing contemporaneously Dan Merkur, "Stages of Ascension in Hermetic Rebirth"].

As a divine fountain of writing, Hermes Trismegistus was credited with tens of thousands of writings, of immense antiquity and high standing. Plato's Timaeus and Critias state that in the temple of Neith at Sais, there were secret halls containing historical records which had been kept for 9,000 years. Clement of Alexandria was under the impression that the Egyptians had forty-two sacred writings by Hermes, encapsulating all the training of Egyptian priests.

Siegfried Morenz has suggested (Egyptian Religion) "The reference to Thoth's authorship...is based on ancient tradition; the figure forty-two probably stems from the number of Egyptian nomes, and thus conveys the notion of completeness." The Neo-Platonic writers took up Clement's "forty-two essential texts".

The so-called "Hermetic literature", the Hermetica is a category of papyri containing spells and induction procedures. In the dialogue called the Asclepius (after the Greek god of healing) the art of imprisoning the souls of demons or of angels in statues with the help of herbs, gems and odors, is described, such that the statue could speak and prophesy. In other papyri, there are other recipes for constructing such images and animating them, such as when images are to be hollow so as to enclose a magic name inscribed on gold leaf.
During the Middle Ages and the Renaissance, the writings attributed to Hermes Trismegistus, known as the Hermetica enjoyed great credit and were popular among alchemists. The "hermetic tradition" therefore refers to alchemy, magic, astrology and related subjects. The texts are usually distinguished in two categories the "philosophical" and "technical" hermetica. The former deals mainly with issues of philosophy, and the latter with magic, potions and alchemy. Among other things there are spells to magically protect objects, hence the origin of the term "Hermetically sealed".

The texts that were traditionally written at the dawn of time, the classical scholar Isaac Casaubon in De Rebus sacris et ecclesiaticis exercitiones XVI (1614) showed, by the character of the Greek, to be more recent: most of the "philosophical" Corpus Hermeticum can be dated to around AD 300.

Modern occultists continue to suggest that some of these texts may be of Pharaonic origin, and that "the forty two essential texts" that contained the core work of his religious beliefs and his life philosophy remain hidden away in a secret library.

In some of the readings of Edgar Cayce, Hermes or Thoth was an engineer from the submerging Atlantis and that he built or designed or directed the construction of the Pyramids of Egypt. Hermes Trismegistus is said to be an incarnation of Jesus.

Within the occult tradition, Hermes Trismegistus is credited with several wives, and more than one son who took his name, as well as more than one grandson. This repetition of given name and surname throughout the generations may at least partially account for the legend of his longevity, especially as it is believed that many of his children pursued careers as priests in the religion he started.