The production of life, both of one’s own in labour and of fresh life in procreation, now appears as a double relationship: on the one hand as a natural, on the other as a social relationship. By social we understand the co-operation of several individuals, no matter under what conditions, in what manner and to what end. It follows from this that a certain mode of production, or industrial stage, is always combined with a certain mode of co-operation, or social stage, and this mode of co-operation is itself a “productive force”. Further, that the multitude of productive forces accessible to men determines the nature of society, hence, that the “history of humanity” must always be studied and treated in relation to the history of industry and exchange.