Plutarch, De Iside et Osiride
Goodwin, Ed.

But their kings (you must know) were always chosen either out of the priesthood or soldiery, the latter having the right of succession by reason of their military valor, [p. 72] and the former by reason of their wisdom. But he that was chosen out of the soldiery was obliged immediately to turn priest, and was thereupon admitted to the participation of their philosophy, whose genius it was to conceal the greater part in tales and romantic relations, containing dark hints and resemblances of truth; which it is plain that even themselves would insinuate to us, while they are so kind as to set up Sphinxes before their temples, to intimate that their theology contained in it an enigmatical sort of learning. Moreover, the temple of Minerva which is at Sais (whom they look upon as the same with Isis) had upon it this inscription: I am whatever was, or is, or will be; and my veil no mortal ever took up. Besides, we find the greater part to be of opinion that the proper name of Jupiter in the Egyptian tongue is Amun (from which we have derived our word Ammon). But now Manetho the Sebennite thinks this word signifies hidden and hiding; but Hecataeus of Abdera saith, the Egyptians use this word when they call anybody; for that it is a term of calling. Therefore they must be of the opinion that the first God is the same with the universe; and therefore, while they invoke him who is unmanifest and hidden, and pray him to make himself manifest and known to them, they cry Amun. So great therefore was the piety of the Egyptians' philosophy about things divine.