The Emerald Tablet of Hermes, is a heretical text purporting to reveal the secret of the primordial substance and its transmutation. This short and cryptic text contains knowledge about the "One Thing" which was highly regarded by early alchemists as the foundation of their art--the first principal of alchemy.

The most widely known text of this ancient tablet are:
It is true without lie, certain and without doubt,
that what is below is like what is above,
and what is above is like what is below,
to accomplish the miracles of the One Thing.

SOURCE(S): Zork: Nemesis (game, design document)