Any adequate account of morality must recognize the difference between the realm of facts and the realm of morals. Sense experience is the source of all that can be known about facts, but values of any kind can be known only through the feelings. Since any given sense experience is generally regarded as having the same meaning for all people, it is possible for one person's conclusions relative to the facts to be verified by others. Feelings, on the other hand, are private and individual, and the way one person feels about a given type of conduct is not necessarily the same as another person will feel about it. Consideration of this point is what has led some of Hume's critics to charge that he was a skeptic in moral philosophy and identified good conduct with anything an individual might feel like doing.
The charge was, however, an unwarranted one. It could be supported only if it could be shown that there is no common element in the way human beings feel about certain types of conduct. Hume believed that there is a common element in the feelings of people just as much as there is a common element in what they experience by means of the sense organs. He insisted that all normal human beings possess a feeling for humanity. This feeling is expressed in the sympathy which one feels toward the happiness and the sufferings of other people. It is this common element in the feelings of people that enables one to speak correctly concerning the principles of morals. If morality were merely a matter of individual feelings in which no common element could be recognized, there would be no principles involved. Principles of morality, although they may exist only in human minds, do remain constant, and since they are applicable to people in general, we must conclude that they are something more than individual feelings.
Any empirical approach to the study of morals must begin with an investigation of those activities which are approved by the vast majority of persons. Accordingly, Hume begins with an analysis of benevolence and justice. These two virtues are selected because there is no type of conduct which is approved more universally than acts of benevolence or of justice in one's treatment of his fellow men. Likewise, it can be said there is general disapproval of injustice and of those actions which serve selfish interests at the expense of the welfare of others.
