“[D]isagreement about the nature of the right and the good is not merely a recurrent phenomenon and not solely the expression of inexperience and error, but the natural outcome of people reasoning freely and conscientiously about how one should live. The realization that moral disagreement is likely to be the outcome when reasonable people discuss among themselves the ultimate questions of life, or when a single person ponders them in his own mind, has been one of the seminal experiences of modernity. “By sowing questions and dividing them up,” Montaigne wrote, “one makes the world flourish and teem with uncertainty and disputes… Difficultatem facit doctrina [learning creates difficulty]… Never have two men judged similarly of the same thing, and it is impossible to find two opinions exactly similar, not only in different men, but in the same man at different times.” Or as Hobbes himself declared (palpably echoing Montaigne), “Divers men, differ not only in their judgment on the senses of what is pleasant, and unpleasant to the taste, smell, hearing, touch, and sight; but also of what is conformable to reason, in the actions of common life. Nay, the same man, in divers times, differs from himself, and one time praiseth, that is, calleth good, what another time he dispraiseth, and calleth evil.”
The important element in this outlook is not the mere realization that opinions vary. That people often disagree about ethical issues is a fact of life known from time immemorial. Aristotle himself began his Nicomachean Ethics with a survey of the competing notions of the good life (pleasure, honor, wealth, virtue, knowledge, and the various combinations of these). What Aristotle did not envision, however, and has become a leading theme in modern thought is that reasonable people, exercising their general capacities of reason in good faith on questions having to do with how the individual should live and society be organized, tend to come to different conclusions – not through any defect of reason, but instead because of their different backgrounds, their different senses of what is salient, their different ways of weighing opposing considerations.It is the realization that reason does not lead naturally to unanimity on these questions that represents the break with the past.”