take the form of relations between the different members; when they are of old standing, formulated in official rules, imposed by a visible authority, maintained by a special set of persons, they occupy so important a place in life, that, to the persons under their influence, they appear as external realities. The men, too, who specialise in an occupation or a function which becomes the dominating habit of their lives, appear as grouped in distinct categories (classes, corporations, churches, governments); and these categories are taken for real existences, or at least for organs of various functions in a real existence, namely, society. We follow the analogy of an animal's body so far as to describe the "structure" and the "functions" of a society, even its "anatomy" and "physiology." These are pure metaphors. By the structure of a society we mean the rules and the customs by which occupations and enjoyments are distributed among its members; by its functions we mean the habitual actions by which each man enters into relations with the others. It may be convenient to use these terms, but it should be remembered that the underlying reality is composed entirely of habits and customs.
The study of institutions, however, obliges us to ask special questions about persons and their functions. In respect of social and economic institutions we have to ask what was the principle of the division of labour and of the division into classes, what were the professions and classes, how were they recruited, what were the relations between the members of the different professions and classes. In respect of political institutions, which are sanctioned by obligatory rules and a visible authority, two new series of questions arise. (1) Who were