His criticism of Marx is really funny. First he makes up a materialist theory of history for himself, which Marx is supposed, in his opinion, to have held, and then he finds something quite different in Marx's works. But from this he does not conclude that he, Barth, has foisted something distorted on to Marx: no, on the contrary, Marx contradicts himself and cannot apply his own theory! Yes, if people could only read! as Marx used to exclaim at criticisms of this kind. I have not got the book here; if I had time I would show you hundreds more absurdities one by one. It is a pity: one sees that the man could accomplish something if he were not so hasty in passing his judgments. It is to be hoped that he will soon write something which will be attacked more; a regular dose of knocking about would do him a lot of good.
Letters: Marx-Engels Correspondence 1891