This social criterion is necessary, not only to mark off studies from one another, but also to grasp the reasons for each study,--the motives in connection with which it shall be presented. How, for example, should we define geography? What is the unity in the different so-called divisions of geography,--mathematical geography, physical geography, political geography, commercial geography? Are they purely empirical classifications dependent upon the brute fact that we run across a lot of different facts? Or is there some intrinsic principle through which the material is distributed under these various heads,--something in the interest and attitude of the human mind towards them? I should say that geography has to do with all those aspects of social life which are concerned with the interaction of the life of man and nature; or, that it has to do with the world considered as the scene of social interaction. Any fact, then, will be geographical in so far as it has to do with the dependence of man upon his natural environment, or with changes introduced in this environment through the life of man. The four forms of geography referred to above represent, then, four increasing stages of abstraction in discussing the mutual relation of human life and nature. The beginning must be social geography, the frank recognition of the earth as the home of men acting in relations to one another. I mean by this that the essence of any geographical fact is the consciousness of two persons, or two groups of persons, who are at once separated and connected by their physical environment, and that the interest is in seeing how these people are at once kept apart and brought together in their actions by the instrumentality of the physical environment. The ultimate significance of lake, river, mountain, and plain is not physical but social; it is the part which it plays in modifying and directing human relationships. This evidently involves an extension of the term commercial. It has to do not simply with business, in the narrow sense, but with whatever relates to human intercourse and intercommunication as affected by natural forms and properties. Political geography represents this same social interaction taken in a static instead of in a dynamic way; taken, that is, as temporarily crystallized and fixed in certain forms. Physical geography (including under this not simply physiography, but also the study of flora and fauna) represents a further analysis or abstraction. It studies the conditions which determine human action, leaving out of account, temporarily, the ways in which they concretely do this. Mathematical geography carries the analysis back to more ultimate and remote conditions, showing that the physical conditions of the earth are not ultimate, but depend upon the place which the world occupies in a larger system. Here, in other words, are traced, step by step, the links which connect the immediate social occupations and groupings of men with the