In two brief epigrams, why Sartre had such a hard time in Critique of Dialectical Reason trying to reconcile his existentialism with Marxism:

“It is not the consciousness of men that determines their being, but, on the contrary, their social being that determines their consciousness.”

~ Karl Marx, Preface to A Contribution to the Critique of Political Economy (1859)

“Freedom is existence, and in it existence precedes essence.”

~ Jean-Paul Sartre, “Existentialism Is a Humanism” (1946)

It is a wonder Sartre so hated psychoanalysis and the unconsciousness for the suggestion that freedom was anything other than absolute, but Marxism he could work with.