Marx-Engels Correspondence 1893: Engels to Franz Mehring
Ideology is a process accomplished by the so-called thinker consciously, indeed, but with a false consciousness. The real motives impelling him remain unknown to him, otherwise it would not be an ideological process at all. Hence he imagines false or apparent motives. Because it is a process of thought he derives both its form and its content from pure thought, either his own or that of his predecessors. He works with mere thought material which he accepts without examination as the product of thought, he does not investigate further for a more remote process independent of thought; indeed its origin seems obvious to him, because as all action is produced through the medium of thought it also appears to him to be ultimately based upon thought. The ideologist who deals with history (history is here simply meant to comprise all the spheres – political, juridical, philosophical, theological – belonging to society and not only to nature), the ideologist dealing with history then, possesses in every sphere of science material which has formed itself independently out of the thought of previous generations and has gone through an independent series of developments in the brains of these successive generations. True, external facts belonging to its own or other spheres may have exercised a co-determining influence on this development, but the tacit pre-supposition is that these facts themselves are also only the fruits of a process of thought, and so we still remain within that realm of pure thought which has successfully digested the hardest facts.
Related:
Ministry of Law, Justice and Parliamentary Affairs of the People’s Republic of Bangladesh: Marx/Engels Letters (PDF)