Karl Marx's theory of ideology, as outlined in The German Ideology, was primarily a critique of idealist philosophy. When Karl Mannheim picked up the term ideology in the 1930s, he broadened its meaning to the point where it lost its function in the struggle against idealist metaphysics. Claiming to think the problem through to its logical conclusion, the sociology of knowledge went so far as to reconcile the concept of ideology with precisely the speculative thought that historical materialism was originally conceived to displace. In effect, Mannheim's notion of history as the Träger of a unified meaning and his notion of social conflict as the expression of ideal differences composing a harmonious whole contributed to a severe reduction of the critique of idealism. This provoked a drawn-out battle between the Frankfurt School and the sociology of knowledge.
The problem with the new worldview sociology, as Herbert Marcuse, Max Horkheimer, and Theodor Adorno saw it, was that it diluted the message of Marx's materialist approach and veered heavily back toward traditional idealist philosophy. The significance of the intervention of the Frankfurt School lies in the fact that the attack on Mannheim repeated Marx and Friedrich Engels's attack on the Young Hegelians. Like Marx and Engels, Horkheimer and his associates managed to “uncloak [the] sheep, who take themselves and are taken for wolves” and settle their accounts with metaphysics in favor of a thoroughly secular understanding of social contradictions. While it stopped short of framing its critique of metaphysics in Marx's theory of the division between mental and manual labor, which was the real advance of Marx over his antagonist Max Stirner, the Frankfurt School revived the original concept of ideology while demonstrating its rejection of any depoliticization of Marxian theory as well as its deep commitment to the dialectical mode of inquiry.