Now one step farther. When the conditions of production, the social divisions into classes, and the laws of property become fetters to the productive forces, when the conflict of interests condense themselves into class struggles, then comes a period of social revolution. "With the change of the economic foundation the entire immense superstructure is more or less rapidly transformed. In considering such transformations the distinction should always be made between the material transformation of the economic conditions of production which can be determined with the precision of natural science, and the legal, political, religious, aesthetic, or philosophic--in short, ideological forms in which men become conscious of this conflict and fight it out. Just as our opinion of an individual is not based on what he thinks of himself, so we are not able to judge of such a period of transformation by its own consciousness; on the contrary, this consciousness must rather be explained from the contradictions of material life, from the existing conflict between the social forces of production and the conditions of production."--(Preface to "Critique of Political Economy.") The revolutionary period only closes when the social order that had become full of contradictions liberates the productive forces and strikes off their fetters, and creates new conditions of production which correspond to them. The old society, which is doomed to disappear, evolves the new conditions of existence before it sinks into oblivion. The men who assist the progress of the new society accordingly occupy themselves with problems which they are able to solve, as the means thereto are given in the material development. Such problems are set before them because, regarded from the theoretical standpoint, they are the mental reflex of the contradictions and revolutionary tendencies within society. Accordingly, the essence of the historical development of human society has been so far the progressive dialectical unfolding and perfection of the productive forces. "In broad outlines," says Marx, "we can designate the Asiatic, the ancient, the feudal, and the modern bourgeois methods of production, as so many epochs in the progress of the economic formation of society. The bourgeois relations of production are the last antagonistic form of the social process of production--antagonistic not in the sense of individual antagonism, but of one arising from conditions surrounding the life of individuals in society; at the same time the productive forces developing in the womb of bourgeois society create the material conditions for the solution of that antagonism. This social formation constitutes, therefore, the closing chapter of the prehistoric stage of human society."