Engels said that “interaction is the real ultimate cause of things”，including the following two situations：one is the interaction of various factors inside an individual thing abstracted from the universally connected whole，which is the real development of this individual thing The ultimate reason；the other is the interaction of the internal elements of things that are in a basic position in the universally connected whole，which is the real ultimate reason for the overall development of this thing. This article mainly examines the second situation. Since the interaction between the various elements in a basic thing is the real ultimate cause of the overall development of this thing，it is impossible to trace the reasons for its development outside the basic thing. For example，in the structure of human society，material production is the foundation of social existence and development，and the interaction between productivity and production relations is the true ultimate cause of social historical development. Therefore，it is impossible to trace the reasons for the development of productive forces in addition to material production. Doing so will fall into the metaphysical way of thinking of “infinite evil”. This article first introduces Plekhanov’s theoretical sources of historical materialism and Marx’s process of establishing historical materialism and exploring the ultimate cause of historical development，and then examines and analyzes Plekhanov’s pros and cons in exploring the ultimate cause of historical development.，Finally，use a dialectical method of thinking to explain that the reasons for the development of productivity cannot be traced beyond the interaction between productivity and production relations.