Apparently I missed a post on the C4SIF site last March, claiming that Proudhon was an advocate of intellectual property. Now, as I am a notorious softy on that question (or self-serving reactionary, depending on who you ask), I’m less inclined to “pistols at dawn” than some might be, but it doesn’t sound much like the Proudhon I know. You can check the comments for some discussion with Stephan Kinsella about the question, which is rendered more difficult because the text at issue is from the half-translated and notoriously difficult System of Economic Contradictions.
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New Proudhon Library
P.-J. Proudhon, Correspondence related to the Studies in Popular Philosophy
Under the general title of Popular Philosophy, I begin an indefinite series of publications on all sorts of subjects, history, literature, political economy, morals, biography, etc., men and things. All this judged, appreciated, explained, interpreted with the aid of the new philosophical principle, the highest and most fruitful, at once objective and subjective, idea and sentiment, law of man and law of nature, justice. Give me five years of this popularization, and I dare say that the public, today tired, disgusted, skeptical, will again take courage and conceive what a philosophical system is, a kind of encyclopedia, whose principle, law, method, end, means, is right.

Bakunin Library
Bakunin, “To the Brothers of the Alliance in Spain” (1872)
Marx, as a thinker, is on the right track. He has established as a principle that all the political, religious and legal evolutions in history are not causes, but effects of the economic evolutions. It is a great and productive thought, that he has not absolutely invented: it has been glimpsed, expressed in part, by many others than him; but finally, to him belongs the honor of having solidly established it and having posited it as the basis of his whole economic system. On the other hand, Proudhon understand and felt liberty beaucoup much better than him—Proudhon, when he did not engage in doctrine and metaphysics, had the true instinct of the revolutionary—he adored Satan and he proclaimed an-archy. It is quite possible that Marx could raise himself theoretically to an even more rational system of liberty than Proudhon—but he lacks Proudhon’s instinct.