Hermeticism, also called Hermetism is a religious and esoteric tradition based primarily upon writings attributed to Hermes Trismegistus ("thrice-greatest Hermes"). These writings have greatly influenced the Western esoteric tradition and were considered to be of great importance during both the Renaissance and the Reformation. The tradition traces its origin to a prisca theologia, a doctrine that affirms the existence of a single, true theology that is present in all religions and that was given by God to man in antiquity.

Much of the importance of Hermeticism arises from its connection with the development of science during the time from 1300 to 1600 AD. The prominence that it gave to the idea of influencing or controlling nature led many scientists to look to magic and its allied arts (e.g., alchemy, astrology) which, it was thought, could put Nature to the test by means of experiments. Consequently, it was the practical aspects of Hermetic writings that attracted the attention of scientists. Isaac Newton placed great faith in the concept of an unadulterated, pure, ancient doctrine, which he studied vigorously to aid his understanding of the physical world

    In Hermeticism the ultimate reality is called by many names, such as God, Lord, Father, Mind (Nous), the Creator, the All, the One, etc. Peculiar to the Hermetic view of the divinity is that all created things pre-exist in God, and God is the nature of the cosmos (being both the substance from which it proceeds and the governing principle which orders it), yet the things themselves and the cosmos were all created by God. Thus, God ('the All') creates itself, and is both transcendent (as the creator of the cosmos) and immanent (as the created cosmos). These ideas are closely related to the cosmo-theological views of the Stoics.

     Hermeticists believe in a prisca theologia, the doctrine that a single, true theology exists, that its elements exist in all religions, and that it was given by God to man in antiquity. In order to demonstrate the truth of the prisca theologia doctrine, Christians appropriated the Hermetic teachings for their own purposes. By this account, Hermes Trismegistus was (according to the fathers of the Christian church) either a contemporary of Moses or the third in a line of men named Hermes—Enoch, Noah, and the Egyptian priest-king who is known to us  us as Hermes Trismegistus

     "As above, so below" is a popular modern paraphrase of the second verse of the Emerald Tablet (a compact and cryptic text attributed to Hermes Trismegistus) as it appears in its most widely divulged medieval Latin translation: Quod est superius est sicut quod inferius, et quod inferius est sicut quod est superius. That which is above is like to that which is below, and that which is below is like to that which is above.