When a historian proceeds to study a selected fragment of the past he has at his disposal, among other things, a specified general knowledge of the process of history and a more detailed knowledge of the problems of the period and the region under investigation. That general knowledge of his, whatever its standard may be and however it helps him in his research, is one of the major elements of what is termed a historian's non-source-based knowledge. Its structure and functions will be investigated in greater detail in Part Four, when procedures in historical research will be studied.