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Philosophy didn't become irrational on its own. It took Karl Marx's work to thoroughly debunked it as such (e.g. Manuscripts of 1844). At least the action and moral parts. The knowledge aspect remained open to speculation and reflection (i.e. epistemology). What they have in common is the successful negation of Marx's work by placing philosophy on life-support, which it still is today, and by acting as if either the marxist critique never happened, or, it is equivalent to leninist/stalinist ideology.