Aristotle defines an organism as that in which the part exists for the sake of the whole. It is not that the existence of the part depends on the existence of the whole, for every whole exists only as an aggregate of parts related in a certain way; but that the shape and nature of the part are determined by the wants of the whole. Thus the shape and nature of my foot are what they are, not for the sake of my foot itself, but for the sake of my whole body, and because it wants to move about. That which the part has to do for the whole is called its function. Thus the function of my foot is to support me, and assist in locomotion. Not all the nature of the part is necessarily for the sake of the whole: the comparative callosity of the skin of my sole is for the protection of my foot itself.
Society is an organism, and man in society is part of an organism according to this definition, in so far as some portion of the nature of man is what it is for the sake of the whole--society. Now conscience is such a portion of the nature of man, and its function is the preservation of society in the struggle for existence. We may be able to define this function more closely when we know more about the way in which conscience tends to preserve society.
Next let us endeavor to make precise the meaning of the words community and society. It is clear that at different times men may be divided into groups of greater or less extent--tribes, clans, families, nations, towns. If a certain number of clans are struggling for existence, that portion of the conscience will be developed which tends to the preservation of the clan; so, if towns or families are struggling, we shall get a moral sense adapted to the advantage of the town or the family. In this way different portions of the moral sense may be developed at different stages of progress. Now it is clear that for the purpose of the conscience the word community at any time will mean a group of that size and nature which is being selected or not selected for survival as a whole. Selection may be going on at the same time among many different kinds of groups. And ultimately the moral sense will be composed of various portions relating to various groups, the function or purpose of each portion being the advantage of that group to which it relates in the struggle for existence. Thus we have a sense of family duty, of municipal duty, of national duty, and of duties toward all mankind.