"…what distinguishes [the Marxist notion of class] from the sociological notion of class is that, for the former, class is precisely a differential concept, that each class is at once a way of relating to and of refusing the others. Whatever its philosophical presuppositions, the sociological view is formally wrong to the degree that it allows us to think of the individual classes in a kind of isolation from each other, with the almost physical separation of social groups in city or countryside, or as “cultures” somehow self-developing and independent from each other: for the notion of the isolated class or social group is just as surely a hypostasis as the notion of the solitary individual in eighteenth-century philosophy. In history also there are no substances tranquilly persevering in their own essence, but rather a relationality and struggle of every instant, in which class is no more free than the individual not to be engaged. So it is that each class implies the existence of all the others in its very being, for it defines itself against them and survives and perpetuates itself only insofar as it succeeds in humiliating its adversaries. Thus, to use the convenient but of course very abstract tripartite formula, the bourgeois defines himself as a nonnoble and a nonworker at the same time, or better still, as an antinoble and an antiworker all in one. And with such a relational concept of class, the criterion of genuinely Marxist analysis is given also: it will necessarily imply the shock of demystification in its very structure, it always in one way or another presuppose a movement from an apparently systematic and intellectually coherent, self-contained surface to that historical situation behind it, in terms of which the ideological product under examination suddenly proves to have had a function and strategic value as a weapon of a determinate kind in a concrete and local struggle. "
Fredric Jameson, Marxism and Form: Twentieth-Century Dialectical Theories of Literature, pg.380-1