Social science deals with one sector of the activity of human beings. Basically, activity can be considered to be any expenditure of energy of any part of the organism. It includes the biochemical processes which go on in the body. But social science does not deal with biochemical processes as such; they are in the domain of physiology. Since social science does not deal with activity in all its forms, abstraction is necessary. The sector of activity which is abstracted for study can be called by any convenient term. The crucial problem of social science is to develop these principles — to develop, in other words, the principles of action. In the social sciences relationships are usually stated in terms of the language of the person who describes them. The implications of this should be carefully examined.