Gregory of Nazianzus

[God gave Adam] a law as a material for his free will to act on. This law was a commandment as to what plants he might partake of and which one he might not touch. This latter was the tree of knowledge; not, however, because it was evil from the beginning when planted, nor was it forbidden because God grudged it to us—let not the enemies of God wag their tongues in that direction or imitate the serpent. But it would have been good if partaken of at the proper time. The tree was, according to my theory, contemplation, which is safe only for those who have reached maturity of habit to enter upon, but which is not good for those who are still somewhat simple and greedy, just as neither is solid food good for those who are yet tender and have need of milk.