The dialectic of the Enlightenment is a philosophy of history that operates as the foundation of Critical Theory. Horkheimer and Adorno wonder why humanity instead of entering a truly human state, led to a kind of barbarism. The answer to this question is revealed in the triumph of a new form of rationality (previously studied by Max Weber) that will receive different denominations: subjective, instrumental (Horkheimer); subjective or identifying (Adorno), one-dimensional (Marcuse); instrumental or strategic (Habermas).
The human effort to dominate nature (which includes man himself) is the central feature of the story of enlightenment. This idea revises the Marxist postulate that places class struggle as the engine of history. For critical theorists, it will animate the story the conflict man nature faced as subject-object. In this effort it was that humanity granted triumph to subjective or instrumental reason, that is, a form of rationality that sun seeks that the media adapt to the subjective interests of self-preservation.
The subjective rationality, serves only the media, but not the ends. Because they consider themselves indifferent and their choice is due to irrational predilections. Subjective reason is only an instrument since it is not used to judge the acts and way of life of man. Then reason, instead of liberating, has led man to barbarism: nature (even human) has “been cosified”. Mastering nature is thus an end in itself.
Horkheimer will say that the illustration suppressed and forgot objective reason, that is, a rationality inherent in reality itself that allowed the determination of desirable ends and ideals. But later, he will think that in relaity, it is not about taking part in one of the forms of rationality, since both lead to excesses. Objective reason leads to the affirmation of illusory values and to the creation of reactionary ideologies... subjective reason will lead to vulgar materialism and even the barbarity of domination. Self-criticism of reason should aim to overcome the limitations of both extremes in a dialectical synthesis.