Plutarch De Iside et Osiride 3

[3] Moreover, many writers have held her to be the daughter of Hermes, ¹ and many others the daughter of Prometheus, ² because of the belief that Prometheus is the discoverer of wisdom and forethought, and Hermes the inventor of grammar and music. For this reason they call the first of the Muses at Hermopolis Isis as well as Justice: for she is wise, as I have said, ³ and discloses the divine mysteries to those who truly and justly have the name of bearers of the sacred vessels and wearers of the sacred robes. These are they who within their own soul, as though within a casket, bear the sacred writings about the gods clear of all superstition and pedantry; and they cloak them with secrecy, thus giving intimations, some dark and shadowy, some clear and bright, of their concepts about the gods, intimations of the same sort as are clearly evidenced in the wearing of the sacred garb. ⁴ For this reason, too, the fact that the deceased votaries of Isis are decked with these garments is a sign that these sacred writings accompany them, and that they pass to the other world possessed of these and of naught else. It is a fact, Clea, that having a beard and wearing a coarse cloak does not make philosophers, nor does dressing in linen and shaving the hair make votaries of Isis; but the true votary of Isis is he who, when he has legitimately received what is set forth in the ceremonies connected with these gods, uses reason in investigating and in studying the truth contained therein.

[3] Moreover, many have reported her the daughter of Hermes, and many of Prometheus; the latter of which they esteem as the author of wit and forecast, and the former of letters and music. For the same reason also they call the former of the Muses at Hermopolis at the same time Isis and Justice, Isis being (as we before said) no other than wisdom, and revealing things divine to such as are truly and justly styled the sacred bearers, and keepers of the sacred robes; and these are such as have in their minds, as in an ark, the sacred doctrine about the Gods, cleansed from superstitious frights and vain curiosities, keeping out of sight all dark and shady colors, and exposing to sight the light and gay ones, to insinuate something of the like kind in our persuasion about the Gods as we have represented to us in the sacred vestments. Wherefore, in that the priests of Isis are dressed up in these when they are dead, it is a token to us that this doctrine goes with them to the other life, and that nothing else can accompany them thither. For as neither the nourishing of beards nor the wearing of mantles can ren der men philosophers, so neither will linen garments or shaved heads make priests to Isis; but he is a true priest of Isis, who, after he hath received from the laws the representations and actions that refer to the Gods, doth next apply his reason to the enquiry and speculation of the truth contained in them.