The causal determination of one category by another is for Marxism, as for all philosophy, a means of establishing causal relationships in domains which lie outside our experience. The categories with which Marxism works have been deduced from real phenomena by an abstraction which has been at least thrice repeated. The categorical thinking of the Marxists always moves in couples. Thus, for the Marxists, the social revolution very closely resembles the movement of mechanical forces, such as results from the collision of two bodies moving in opposite directions. The class struggle as a categorical conflict between a force identified with capitalism, and another force identified with socialism, is a concept whose function it is to give a directive to our feelings. A different affective state, a social will directed towards a different end, will decompose the conceptual image of the same real universe into different categorical elements.