“The fact is that if the writer's attention is on producing a work of art, a work that is good in itself, he is going to take great pains to control every excess, everything that does not contribute to this central meaning and design. He cannot indulge in sentimentality, in propagandizing, or in pornography and create a work of art, for all these things are excesses. They call attention to themselves and distract from the work as a whole.”