(1) We may compare together similar categories of special facts, language, religions, arts, governments, taking them from the whole of humanity, and classifying together those which most resemble each other. We obtain families of languages, religions, and governments, which we may again classify and arrange among themselves. This is an abstract kind of classification; it isolates one species of facts from all the others, and thus renounces all claim to exhibit causes. It has the advantage of being rapidly performed and of yielding a technical vocabulary which is useful for designating facts. (2) We may compare real groups of real individuals, we may take societies which figure in history and classify them according to their similarities. This is a concrete classification analogous to that of zoology, in which, not functions, but whole animals are classified. It is true that the groups are less clearly marked than in zoology; nor is there a general agreement as to the characteristics in respect of which we are to look for resemblances. Are we to choose the economic or the political organisation of the groups, or their intellectual condition? No principle of choice has as yet become obligatory. History has not yet succeeded in establishing a scientific system of comprehensive classification. Possibly human groups are not sufficiently homogeneous to furnish a solid basis of comparison, and not sharply enough divided to be treated as comparable units. VIII. The study of the relations between simultaneous facts consists in a search for the connections between all the facts of different species which occur in a given society. We have a vague consciousness that the different habits which are separated by abstraction and ranged under different categories (art, religion, political institutions), are not isolated in reality, that they have common characteristics, and that they are closely enough connected for a change in one of them to bring about a change in another. This is a fundamental idea of the _Esprit des Lois_ of Montesquieu. This bond of connection, sometimes called _consensus_, has received the name of _Zusammenhang_ from the German school. From this conception has arisen the theory of the _Volksgeist_ (the mind of a people), a counterfeit of which has within the last few years been introduced into France under the name of "âme nationale." This conception is also at the bottom of the theory regarding the soul of society which Lamprecht has expounded. After the rejection of these mystical conceptions there remains a vague but incontrovertible fact, the "solidarity" which exists between the different habits of one and the same people. In order to study it with precision it would be necessary to analyse it, and a connecting bond cannot be analysed. It is thus quite natural that this part of social science should have remained a refuge for mystery and obscurity.