2. If any one held that the natural law of conscience was natural in the same way as the sense of temperature: if one held to the existence of a Moral Sense in all men, settling questions of right and wrong, as surely as all men know sweet things from bitter by tasting them: these stories, and they could be multiplied by hundreds, abundantly suffice to confute the error. There is no authentic copy of the moral law, printed, framed, and hung up by the hand of Nature, in the inner sanctuary of every human heart. Man has to learn his duties as he learns the principles of health, the laws of mechanics, the construction and navigation of vessels, the theorems of geometry, or any other art or science. And he is just as likely to go wrong, and has gone wrong as grievously, in his judgments on moral matters as on any other subject of human knowledge. The knowledge of duties is _natural_ (as explained in the previous section, n. 2), not because it comes spontaneously, but because it is necessary to our nature for the development and perfection of the same. Thus a man _ought_, so far as he can, to learn his duties: but we cannot say of a man, as such, that he _ought_ to learn geometry or navigation. If a man does not know his duties, he is excused by ignorance, according to the rules under which ignorance excuses (c. iii., s. i., nn. 3-5, p. 27). If a man does not know navigation, there is no question of _excuse_ for what he was not bound to learn, but he may suffer _loss_ by his want of knowledge.