There were two elements, indeed, in the system of popular ethics inherited from the last century, which were long thought incompatible with its complete reduction to the form of a physical science. These were the doctrines of free-will and of a moral sense. Each, however, was understood in a way which suggested to the naturalist a ready explanation of its supposed claim to lie beyond his sphere. The moral sense, according to the accepted view, was a specific susceptibility to pleasure or pain in the contemplation of certain acts. What was the quality in the acts which excited this pleasure or pain in the contemplation of them? If it were something in the conception of which any originative function of the reason was implied, then the existence of the moral sense would have meant that there was a determining agent in the inner life of man, of which no natural history could be given. But those writers who had made most of the moral sense had been very indefinite in their account of the quality in action to which it was relative. The most consistent theory on the subject was Hume’s. According to him the pleasure of moral sense is pleasure felt in the mere survey of an act, independently of any consequences of the act to the person contemplating it; and that which occasions this pleasure is the tendency of the act to bring pleasure to the agent himself or to others. Moral sense, in short, is a social sentiment either of satisfaction in the view of such conduct as has been generally found to increase the pleasure or diminish the pain of others, or of uneasiness in the reverse, quite apart from any expectation of personal advantage or loss. It is thus properly not by the action of the person feeling it, but by that of others, that it is excited. An act of a man’s own, necessarily proceeding, according to Hume, from some desire for pleasure which it satisfies or fails to satisfy, must have personal consequences for him, incompatible with that disinterested survey which alone yields the pleasure or pain of moral sense, properly so called. Sympathy, however, with the effect which he knows that his act produces on the moral sense of others, may modify the feeling which it causes to the doer of it. An act, in gratification of some passion, which he would otherwise look forward to as pleasant, may become so painful in anticipation from sympathy with the general uneasiness which he knows would arise upon the contemplation of it that, without any fear of punishment, he abstains from doing it. (§3 ¶1)