It is not the consciousness of men that determines their existence, but, on the contrary, their social existence determines their consciousness. At a certain stage of their development, the material forces of production in society come into conflict with the existing relations of production, or - what is but a legal expression for the same thing - with the property relations within which they had been at work before. From forms of development of the forces of production these relations turn into their fetters. Then comes the period of social revolution. With the change of the economic foundation the entire immense superstructure is more of 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 the conflict and fight it out.
-Karl Marx and Fredrick Engels, The German Ideology