Social anthropology, defined operationally in terms of what social anthropologists have done in the last ¬fty years, is the study and comparison of tribal societies and of small ¬elds of social life with emphasis on the role of custom. When a social anthropologist's research leads him into any ¬eld which belongs to other disciplines, what line should he adopt? What use may he make of the results that other scholars have already achieved? Must he knowingly make naive assumptions concerning events which they have regarded as complex?