The reference is to Gen. ii. 9 (in LXX.), where the tree is called, τὸ ξύλον τοῦ εἰδέναι γνωστὸν καλοῦ καὶ πονηροῦ. S. Gregory proceeds to ascertain the exact meaning of the word γνωστὸν in the text; the eating is the “knowing,” but what is “knowing”? He answers, “desiring.”