"I often quote concepts, texts and phrases from Marx, but without feeling obliged to add the authenticating label of a footnote with a laudatory phrase to accompany the quotation. As long as one does that, one is regarded as someone who knows and reveres Marx, and will be suitably honoured in the so-called Marxist journals. But I quote Marx without saying so, without quotation marks, and because people are incapable of recognising Marx's texts I am thought to be someone who doesn't quote Marx. When a physicist writes a work of physics, does he feel it necessary to quote Newton and Einstein?"
http://foucauldians.blogspot.com/2007/08/foucault-on-marx.html