Historians of old relate that the great Temple of Apollo at Delphi had several inscriptions on its entrance. Most prominent was a large letter E (Epsilon) made of gold. Its meaning was an enigma, even in antiquity.
Over the millennia, many interpretations have been suggested. Some are based on the meaning of E, fifth letter of the Greek alphabet, as the number five. Others say it is not an letter at all, but a religious symbol, or glyph, which predates writing.
Plutarch, the eminent biographer and Middle Platonist philosopher, was, in addition, a priest at Delphi. He suggested that the meaning is found not in the letter itself, but in its pronunciation: "EI", which in Greek means "Thou art." Hence he speculated it served as a respectful address to the god, Apollo -- as though, when entering the temple, one honors the god by saying "Thou art!"
A still more intriguing possibility is that "EI" is the god's greeting to visitors. That is, the god of revelation, Apollo, greets the seeker, saying: "Know, O soul, this most profound and mysterious truth, beside which all else fades to insignificance: THOU ART!"