Ideology is a process which is indeed accomplished consciously by the so-called thinker, but it is the wrong kind of consciousness. The real motive forces impelling him remain unknown to the thinker; otherwise it simply would not be an ideological process. Hence he imagines false or illusory motive forces. Because it is a rational process he derives its form as well as its content from pure reasoning, either his own or that of his predecessors. He works exclusively with thought material, which he accepts without examination as something produced by reasoning, and does not investigate further for a more remote source independent of reason; indeed this is a matter of course to him, because, as all action is mediated by thought, it appears to him to be ultimately based upon thought.
The historical ideologist (historical is here simply a comprehensive term comprising political, juridical, philosophical, theological – in short, all the spheres belonging to society and not only to nature) thus possesses in every sphere of science material which has arisen independently out of the thought of previous generations and has gone through its own independent course of development in the brains of these successive generations. True, external facts belonging to one or another sphere may have exercised a co-determining influence on this development, but the tacit presupposition is that these facts themselves are also only the fruits of a process of thought, and so we still remain within that realm of mere thought, which apparently has successfully digested even the hardest facts.
It is above all this semblance of an independent history of state constitutions, of systems of law, of ideological conceptions in every separate domain that dazzles most people. If Luther and Calvin “overcome” the official Catholic religion, or Hegel “overcomes” Fichte and Kant, or Rousseau with his republican Contrat social indirectly “overcomes’ the constitutional Montesquieu, this is a process which remains within theology, philosophy or political science, represents a stage in the history of these particular spheres of thought and never passes beyond the sphere of thought. And since the bourgeois illusion of the eternity and finality of capitalist production has been added to this, even the overcoming of the mercantilists by the physiocrats and Adam Smith is regarded as a sheer victory of thought; not as the reflection in thought of changed economic facts but as the finally achieved correct understanding of actual conditions subsisting always and everywhere – in fact, if Richard Coeur-de-Lion and Philip Augustus had introduced free trade instead of getting mixed up in the crusades we should have been spared five hundred years of misery and stupidity.
Engels to Franz Mehring in Berlin; London, July 14, 1893, Marx Engels, Selected Correspondence, Progress Publishers, Moscow, 1982, 434-435