Of the word ΕΙ engraven over the gate of Apollo's temple at Delphi (17)

But Ammonius, who had himself also bestowed not the worst part of his time in mathematical philosophy, was delighted with what had been spoken, and said: It is not meet too eagerly to oppose these young men about these things, except by saying that every one of the numbers will afford you, if you desire to praise it, no small subject of commendations. And what need is there to speak of others? For the septener, sacred to Apollo, will take up a day’s time, before one can in words run through all its powers. We shall therefore pronounce, that the Sages do at once contest both against common law and a long series of time, if, throwing the septenary out of its seat, they consecrate the quinary to the God, as being more suitable to him. I am therefore of opinion, that this syllable signifies neither number, order, nor connection, nor any other of the deficient parts, but is a self-perfect appellation and salutation of the God, which brings the speaker to the conception of the power of the God at the very moment of uttering it. For the God in a manner calls upon every one of us who comes hither, with this salutation, Know thyself, which is nothing inferior to All hail. And we again, answering the God, say to him Εἶ, thou art; attributing to him the true, unfeigned, and sole appellation of being, as agreeing to him alone.