James Guillaume, in the "Biographical Notice" in his French edition of Bakunin's Works, includes part of an 1870 manuscript written by Bakunin on the subject of Proudhon and Marx:
Proudhon, despite all his efforts to shake off the traditions of classical idealism, nonetheless remained all his life an incorrigible idealist, inspired, as I told him two months before his death, sometimes by the Bible, sometimes by Roman law, and always a metaphysician to the fingertips. His great misfortune is that he never studied the natural sciences, and he never adopted its methods. He had some instincts of genius that made him glimpse the right path, but, led by the bad idealist habits of his mind, he always fell back into the old errors: so that Proudhon has been a perpetual contradiction, — a vigorous genius, a revolutionary thinker always struggling against the phantoms of idealism, but never managing to vanquish them.
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 had understand and felt liberty much better than him. Proudhon, when he did not engage in doctrine and metaphysics, had the true instinct of a revolutionary: he adored Satan and he proclaimed an-archy. It is very possible that Marx could theoretically raise himself a still more rational system of liberty than Proudhon, but he lacks the instinct of liberty: he is, from head to foot, an authoritarian. [The full manuscript letter from which this was taken is now available on the Bakunin Library blog.]
This is obviously the sort of "damning with faint praise" that has been Proudhon's legacy among many anarchists. But it raises all sorts of questions. It is tempting to think that, despite rejecting some aspects of Proudhon's economic analysis (though Bakunin seems to credit Proudhon with "glimpses of the right path" at best), the anarchist collectivists at least inherited the theory of collective force and collective persons from Proudhon's mutualism. But could Bakunin really have adopted even this much from Proudhon, and still dismissed him as "always a metaphysician to the fingertips"? The relationship between metaphysics and the natural sciences was a fairly important issue to Proudhon, and he certainly seems to have thought he had adopted natural-scientific methods, at least where they were appropriate.
Iain McKay has gone to some lengths to paint the collectivist anarchists as fundamentally faithful to Proudhon's thought, except in those cases where they were more faithful to its logics than the mutualists themselves. But what Bakunin actually said about Proudhon's method suggests very little actual engagement with it (and De Paepe was enough more dismissive that the translation work is slow, because it actively annoys and depresses me.) Iain acknowledges that the land-tenure debates in the First International were much more about the future of farming practices than they were about property theory. But that is certainly not the case in the general disagreement between collectivists and communists on one side, and the various forms of mutualists on the other. The "future of farming" debate, together with hand-waving about "metaphysics," has simply served as a substitution for a more serious debate about property, and about the questions of science and method that led Proudhon and the mutualists to different conclusions about a range of issues, starting with, but certainly not limited, to property questions.
As pleased as I am to see Property is Theft! available (and I'm finally getting a chance to settle down with the printed version and see what other translators have provided for it), there is a sense in which even this considerable dose of Proudhon's work is as much a anti-mutualist management of Proudhon's legacies as a celebration of his thought. Reading Iain's continuing comments on my translations, you would never know that I have presented (as have others) a reading of Proudhon's method, and an rather in-depth account of his property theory, that is not subordinated to the evolutionary account, according to which collectivism and then communism "fix" the follies of "reformist" mutualism.