Extension theory is a famous theory in the history of Marxist interpretation. Its core viewpoint is that dialectical materialism is the world outlook of Marxist philosophy, and historical materialism is the result of the 'promotion' and 'application' of dialectical materialism principle in the field of social history. This view is embodied in Stalin's On Dialectical Materialism and Historical Materialism, and has become an authoritative view to explain the relationship between dialectical materialism and historical materialism.