Marx' critique of capitalism as practiced in the early middle 1800s is a powerful commentary. Child labor, 70-hour weeks, company stores etc. were all, ultimately, exploitative to the point of being abusive. Nor was "informed consent" really possible for the labor force of the era.
With his emphasis on the dialectic as the sweeping tool for explaining history, however, Marx falls into a classic trap of academic thinking -- failing to account for change and/or the impact of his own critique. Capitalism, for Marx, had to continue to act as some great ogre and be felled by violence to yield the radiant future. He never really accounted for reasonable people thinking, "yes, there is some abuse and it needs to stop. Now how do we improve things without tossing out the child along with the dirty water." Its a variation on the "either-or" fallacy to assume that things will either change the way/direction you think they should or they will never change. Marx is FAR from alone in this flaw to his thinking. It is a hallmark of academe.
Really, Marx could stand as a poster child for the entire critical project. They can and do expose flaws and gross inequitites in the current modus vivendi, but they generally screw up by the numbers when trying to "answer" the problems identified by their critique.
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