ἰδέαν.  Cyril uses the word in the Platonic sense, as in the next sentence he adopts the formula, which Plato commonly uses in describing the “idea:”  ἀεὶ κατὰ τὰ αὐτὰ καὶ ὡσαύτως ἔχειν.  Phaed. 78 c.