There are necessities and impossibilities in reality which do not obtain in fiction, any more than the law of gravity to which we are subject controls what is represented in a picture. … It is the same with pure good; for a necessity as strong as gravity condemns man to evil and forbids him any good, or only within the narrowest limits and laboriously obtained and soiled and adulterated with evil. … The simplicity which makes the fictional good something insipid and unable to hold the attention becomes, in the real good, an unfathomable marvel.

Morality and literature, pp. 160-161 - On Science, Necessity, and the Love of God (1968)