Here, then, we come to a proof, more conclusive even than that before given, that social actions are regarded by me as resulting from mental factors. Though the specialization of functions, or division of labor, is held to be analogous in living bodies and social bodies—though, in both cases, co-operation of the mutually-dependent parts has to be effected by stimuli conveyed from one to another; yet it is shown that while in the one case this prerequisite is effected by a physical process, it is in the other case effected by a psychical process. So that beyond the proofs variously given that the organization of each society is mainly caused by the mental traits of its units, there is here given the proof that these mental traits produce their results through certain mental products—the signs of feelings and thoughts.
And now let me add a not unimportant conclusion brought into view by this long explanation. In the course of it there has become manifest to me the essential distinction, which I had not before observed, between the dependencies of Sociology on Biology and the dependencies of Sociology on Psychology. They concern respectively the substance of society and the structure of society. We may contemplate the social aggregate simply as a mass of living units, ignoring any arrangement of its parts; or, tacitly positing the existence of the mass, we may occupy ourselves exclusively with the arrangement of its parts. Under the one head we are concerned only with changes of quantity and quality—increases or decreases of the units in number, and organic modifications of their natures: changes produced in the course of generations by subjection of the units to certain conditions of life. For interpreting social phenomena included in this group, we depend directly upon Biology. Under the other head we are concerned only with the development of this social aggregate into an organization of mutually-dependent parts performing different duties the gradual evolution of structures and correlative functions and formation of a more and more integrated whole. For interpreting the phenomena included in this far more conspicuous and important group, we depend directly upon Psychology. Though the two can not be sharply separated, since bodily life and mental life are indissolubly united, and exert reciprocal influences, yet, as being respectively concerned with social substance and social form, the two are sufficiently contrasted.
