Theodor Adorno once remarked that pessimistic thinkers (he had Freud rather than Marx in mind) do more service to the cause of human emancipation than callowly optimistic ones. This is because they bear witness to an injustice which cries out for redemption, and which we might otherwise forget. By reminding us of how bad things are, they prompt us to repair them. They urge us to do without opium.
