Only one point is lacking which Marx and I did not stress systematically enough in our writings and in relation to which we are equally to blame. Namely, we both placed and had to place the chief weight upon the derivation of political, legal and other ideological notions, as well as the actions which they led up to, from fundamental economic facts. In consequence we neglected the formal side, i.e., the way in which these ideas, etc., arose, for the sake of the content. That gave our opponents a welcome occasion for misunderstanding. Paul Barth is a striking example. Ideology is a process which of course is carried on with the consciousness of the so-called thinker but with a false consciousness. The real driving forces which move him, he remains unaware of, otherwise it would :not be an ideological process. He therefore imagines false or apparent driving forces. Because it is a thought process, he derives both its content and form from pure thought, either his own or that of his predecessors. He works with purely conceptual material which he unwittingly takes over as the product of thought and therefore does not investigate its relations to a process further removed from and independent of thought. Indeed this seems to him self-evident, for it appears to him that since all activity is mediated by thought, it is ultimately grounded in thought. The historical ideologist (and historical here simply takes in political, jural, philosophical, theological, in short, all domains which belong to society and not merely to nature) the historical ideologist is confronted in every scientific field by material which has been built up independently out of the thought of earlier generations, and which through the minds of these successive generations has undergone an independent development peculiar to itself. External facts from this or other fields may have contributed to determine this development but these facts, according to the tacit presupposition made, are themselves mere fruits of a thought process. And so we still remain in the realm of pure thought which has succeeded so well in digesting even the toughest facts. It is this appearance of an independent history of state constitutions, systems of law, of ideologies in every special field, which, above all, has blinded so many people. When Luther and Calvin transcend the official Catholic religion; when Hegel transcends Fichte and Kant; and Rousseau, indirectly with his contrat social, the constitutionalist, Montesquieu it is a process which remains within theology, philosophy and political science. It merely represents a stage in the history of these intellectual domains and never emerges from the field of pure thought at .all. And ever since the illusion of the eternity and ultimacy of the system of capitalist production has been added, even the refutation of the Mercantilists by the physiocrats and A. Smith has been regarded not as the intellectual reflection of altered economic realities, but only as a victory of thought, as a correct insight, won at last, into actual conditions existing always and everywhere. If only Richard the Lion-hearted, and Philip Augustus, had introduced free trade, instead of involving themselves in crusades, five hundred years of misery and stupidity would have been spared us. This side of the matter, which I can here only indicate, we have all neglected, I think, more than it deserved. It s the old story. In the beginning the form is always neglected for the content As already said, I myself have made that error and it has always occurred to me only post festum. I am far from reproaching you with it. As an old sinner in this respect I have hardly the right, just the contrary. But I do wish to call your attention to this point for the future. This is bound up with the stupid conception of the ideologists. Because we denied that the different ideological spheres, which play a part in history, have an independent historical development, we were supposed therewith to have denied that they have any historical efficacy. At the basis of this is the ordinary undialectical notion of cause and effect as fixed, mutually opposed, polar relations, and a complete disregard of reciprocity. These gentlemen forget, almost intentionally, that an historical factor, once it has been brought into the world by other ultimately economic facts thereupon also reacts upon its surroundings and even affects its own causes. Thus Barth, e.g., in connection with priesthood and religion, on p.475 in your book ... 
Letters: Marx-Engels Correspondence 1890