“In the same book he became the first man willingly to claim the title of anarchist.”

Anarchism : A History of Libertarian Ideas and Movements (1962)
Context: Like such titles as Christian and Quaker, "anarchist" was in the end proudly adopted by one of those against whom it had been used in condemnation. In 1840, Pierre-Joseph Proudhon, that stormy, argumentative individualist who prided himself on being a man of paradox and a provoker of contradiction, published the work that established him as a pioneer libertarian thinker. It was What Is Property?, in which he gave his own question the celebrated answer: "Property is theft." In the same book he became the first man willingly to claim the title of anarchist.
Undoubtedly Proudhon did this partly in defiance, and partly in order to exploit the word's paradoxical qualities. He had recognized the ambiguity of the Greek anarchos, and had gone back to it for that very reason — to emphasize that the criticism of authority on which he was about to embark need not necessarily imply an advocacy of disorder. The passages in which he introduces "anarchist" and "anarchy" are historically important enough to merit quotation, since they not merely show these words being used for the first time in a socially positive sense, but also contain in germ the justification by natural law which anarchists have in general applied to their arguments for a non-authoritarian society.