Plutarch, De Iside et Osiride
Frank Cole Babbitt, Ed.

As for the robes, those of Isis¹ are variegated in their colours ; for her power is concerned with matter which becomes everything and receives everything, light and darkness, day and night, fire and water, life and death, beginning and end. But the robe of Osiris has no shading or variety in its colour, but only one single colour like to light. For the beginning is combined with nothing else, and that which is primary and conceptual is without admixture ; wherefore, when they have once taken off the robe of Osiris, they lay it away and guard it, unseen and untouched. But the robes of Isis they use many times over; for in use those things that are perceptible and ready at hand afford many disclosures of themselves and opportunities to view them as they are changed about in various ways. But the apperception of the conceptual, the pure, and the simple, shining through the soul like a flash of lightning, affords an opportunity to touch and see it but once.² For this reason Plato³ and Aristotle call this part of philosophy the epoptic⁴ or [p. 183] mystic part, inasmuch as those who have passed beyond these conjectural and confused matters of all sorts by means of Reason proceed by leaps and bounds to that primary, simple, and immaterial principle ; and when they have somehow attained contact with the pure truth abiding about it, they think that they have the whole of philosophy completely, as it were, within their grasp.