Plutarch's treatise περὶ ᾿´Ισιδος καὶ ᾿Οσίριδος is a work of considerable importance not only to the student of later Platonism, but also to the Egyptologist; yet it is a somewhat remarkable fact that it has been much neglected by the latter, although he alone possesses the knowledge that would help to clear up many of the confused and contradictory statements made by Plutarch with regard to Egyptian mythology. The commentaries of Sayce and Wiedemann on the second Book of Herodotus have been invaluable to the historian and mythologist alike; nevertheless, Plutarch's excursus into the realms of Egyptian religious lore has never received the systematic attention of the Egyptologist. It can hardly be said that this is due to the want of importance attached to the subject. It is generally admitted that, whereas the eleventh chapter of the Metamorphoses of Apuleius is our principal source of knowledge concerning the Graeco-Roman cult of Isis, the treatise of Plutarch is almost our only account of the doctrine which the Alexandrian Platonists wove round that goddess and Osiris. There can be little doubt that Plutarch's theories were the same as those generally held by Greek and Roman worshippers of Isis, and the fact that the treatise is addressed to Klea, who appears to have been a professed devotee of the goddess, renders them additionally interesting.