"In the sense used by Marx and Engels, the concept of ideology was intended to mean forms of which prevent people from realising that their thinking about the world is determined by some conditions which do not depend on them and which are not themselves ingredients of consciousness. In ideological thinking, people imagine that the logic of thinking itself rules their consciousness and they are organically incapable of being aware of the social situations and of the interests which mould their mental work. This concept of ideology as false consciousness or as thinking that cannot be aware of its own sources may indeed be useful […] The defect of the concept, however, is that we never have criteria for stating that a certain theory or doctrine does not fall under the concept, even as far as natural science is concerned; nor may we ever be certain that a criticism of ideology is not itself ideological. No conceivable means are available for stating that Capital is not an ideology in this sense. Certainly, Marx maintained (not only in his famous letter to Ruge, but in The Poverty of Philosophy as well […]) that his own theoretical work was to express the real historical movement, i.e. that he was aware of the social sources of his own thinking and that he was in this sense himself free from ideology; however, there is no way of finding out beyond doubt that Marx or that anybody who conceives his own thinking as an "expression" of a certain historical process is not deluding himself about the meaning of his own self-consciousness."