《道德原则研究·第四章·论政治社会》第3节
Those who live in the same family have such frequent opportunities of licence of this kind, that nothing could prevent purity of manners, were marriage allowed, among the nearest relations, or any intercourse of love between them ratified by law and custom. Incest, therefore, being pernicious in a superior degree, has also a superior turpitude and moral deformity annexed to it. What is the reason, why, by the Athenian laws, one might marry a half-sister by the father, but not by the mother? Plainly this: The manners of the Athenians were so reserved, that a man was never permitted to approach the women's apartment, even in the same family, unless where he visited his own mother. His step-mother and her children were as much shut up from him as the woman of any other family, and there was as little danger of any criminal correspondence between them. Uncles and nieces, for a like reason, might marry at Athens; but neither these, nor half-brothers and sisters, could contract that alliance at Rome, where the intercourse was more open between the sexes. Public utility is the cause of all these variations. To repeat, to a man's prejudice, anything that escaped him in private conversation, or to make any such use of his private letters, is highly blamed. The free and social intercourse of minds must be extremely checked, where no such rules of fidelity are established. Even in repeating stories, whence we can foresee no ill consequences to result, the giving of one's author is regarded as a piece of indiscretion, if not of immorality. These stories, in passing from hand to hand, and receiving all the usual variations, frequently come about to the persons concerned, and produce animosities and quarrels among people, whose intentions are the most innocent and inoffensive. To pry into secrets, to open or even read the letters of others, to play the spy upon their words and looks and actions; what habits more inconvenient in society? What habits, of consequence, more blameable? This principle is also the foundation of most of the laws of good manners; a kind of lesser morality, calculated for the ease of company and conversation. Too much or too little ceremony are both blamed, and everything, which promotes ease, without an indecent familiarity, is useful and laudable.
注释摘要
休谟在这一段里要做的事情很明确：他要把乱伦禁忌这种几乎被所有人视为自明之理、近乎本能反感的道德规则，也拉回到效用这个唯一的基础上来解释。上一段他刚讲过贞洁与婚姻忠诚，指出这类德性之所以被看重，完全是因为人类幼年漫长无助、需要父母协作养育后代。现在他要处理一个更棘手的问题——血缘至亲之间的性禁忌。这种禁忌给人的道德直觉太强了，以至于人们很容易以为它根本不依赖…