If a writer regularly treats all life bitterly, scorning love, scorning loyalty, scorning decency (according to some standard)—or, to put it another way, if some writer’s every remark strikes most or many readers as unfair, cruel, stupid, self-regarding, ignorant, or mad; if he has no good to say of anything or anyone except the character who seems to represent himself; if he can find no pleasure in what happy human beings have found good for centuries (children and dogs, God, peace, wealth, comfort, love, hope, and faith)—then it is safe to hazard that he has not made a serious effort to sympathize and understand, that he has not tried to guess what special circumstances would make him behave, himself, as his enemies behave. We discern the same in some more subtle works of art; for instance, when we sense the writer’s refusal to be fair to some one or two minor characters, or to some region he dislikes (usually the American Midwest). Whatever some possible divinity might say of such a writer’s fictions, the non omniscient can say this much: he is not using fiction as a mode of thought but merely as a means of preaching his peculiar doctrine. The more appealing or widely shared the doctrine, the more immoral the book.
John Gardner, On Moral Fiction, via veronimitch