Plutarch, De Iside et Osiride
Goodwin, Ed.

As to the sacred vestments, that of Isis is party-colored and of different hues; for her power is about matter, which becomes every thing and receives every thing, as light and darkness, day and night, fire and water, life and death, beginning and ending. But that of Osiris has no shade, no variety of colors, but one only simple one, resembling light. For the first principle is untempered, and that which is first and of an intelligible nature is unmixed; which is the reason why, after they have once made use of this garment, they lay it up and keep it close, invisible and not to be touched. But those of Isis are used often. For sensible things, when they are of daily use and familiar to us, afford us many opportunities to display them and to see them in their various mutations; but the apprehension of what is intelligible, sincere, and holy, darting through the soul like a flash of lightning, attends but to some one single glance or glimpse of its object. For which reason both Plato and Aristotle call this part of philosophy by the name of the epoptic or mysterious part, intimating that those who by help of reason have got beyond these fanciful, mixed, and various things mount up to that first, simple, and immaterial being; and when they have certainly reached the pure truth about it, they believe they have at last attained to complete philosophy.