Origen

“ The nature of actions—good, bad, and indifferent—has been investigated by the Greeks; and the more successful of such investigators [3] lay down the principle that intention alone gives to actions the character of good or bad, and that all things which are done without a purpose are, strictly speaking, indifferent; that when the intention is directed to a becoming end, it is praiseworthy ”

“ They have said, accordingly, in the section relating to “things indifferent,” that, strictly speaking, for a man to have sexual intercourse with his daughters is a thing indifferent, although such a thing ought not to take place in established communities. And for the sake of hypothesis, in order to show that such an act belongs to the class of things indifferent, they have assumed the case of a wise man being left with an only daughter, the entire human race besides having perished ”