Horkheimer defines true reason as rationality . He details the difference between objective and subjective reason and states that we have moved from objective to subjective. Objective reason deals with universal truths that dictate that an action is either right or wrong.
Subjective reason takes into account the situation and social norms.
The movement from one type of reason to the other occurred when thought could no longer accommodate these objective truths or when it judged them to be delusions. Under subjective reason, concepts lose their meaning. All concepts must be strictly functional to be reasonable. Horkheimer is writing in 1946 and is influenced by Nazi power in Germany. He is also issuing a warning against this happening again.