How Karl Marx dealt with criticisms to his doctrine

Fully aware of his impotence in this regard, Karl Marx resorted to a subterfuge. He and his followers, down to those who called their doctrines "sociology of knowledge," tried to discredit economics by their spurious ideology-concept. As the Marxians see it, in a "class society" men are inherently unfit to conceive theories that are a substantially true description of reality. A man's thoughts are necessarily tainted "ideologically." An ideology, in the Marxian sense of the term, is a false doctrine, which, however, precisely on account of its falsity, serves the interests of the class from which its author stems. There is no need to answer any critique of the socialist plans. It is fully sufficient to unmask the nonproletarian background of its author.

-- Positivism and the Crisis of Western Civilization

Translation: anyone who speaks against communism is wrong and we can dismiss him, on account of him being not a communist.  Ad hominem fallacy through and through.  Not to mention that his notion of "men are unable to conceive theories..." is self-evidently and verifiably false.  And he quite willingly ignored that, if he applied his theory consistently, then Marx's own thoughts are tainted "ideologically" (whatever the hell that means), ergo Marxism itself would automatically be disqualified from being a "theory that is a substantially true description of reality" either -- a paradox no different from asserting that all assertions are false.  So Marx was being either an intellectually dishonest douchebag, or was too stupid to articulate a coherent and valid idea -- there are really no other options.

But people couldn't detect this obvious logical fart.  That this man and his evidently logically invalid and noxious doctrine are regarded highly today is only a testament to the retardation of rationality in the world.