One of the central concerns of the Frankfurt School is the nature of reason and its place in contemporary society. A good example of this is Max Horkheimer's book Eclipse of Reason (Continuum, 1947, 1974). Horkheimer's concern when he wrote this (at the end of WWII) was to account for how the Nazis made their program seem reasonable to the German population. He was interested not just in the ways that we are irrational, but in the ways that our forms of reason pave the way for domination.
Horkheimer argues that there are two senses of reason, subjective and objective. Most people today would locate reason as a subjective faculty of the mind, while people at other times would locate it as a feature of reality.
This means in part that we tend to treat reason as a means-ends calculation. We do not question our desires, but reason becomes an issue when we want to reach those desires. It is important to note that Horkheimer thinks that subjective reason was a major means of dissolving superstition. We cease to believe that there is an external order, dominated by external agency, and start to believe that reason is within us, and that there is no external order. We stop believing in "Reason" and start believing in "reasonable". And, as reason became subjectivized, it also became formalized.
Recognizing the difference between objective and subjective reason is important. When objective reason was the rule, Horkheimer argues that reason was regarded as an instrument for understanding and determining ends. Subjective, formalist reason, on the other hand, is only concerned about means to ends. Objective reason, therefore, is inherent in reality, and it calls for a specific mode of behaviour.