“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.
What is to be the form of government in the future? … I hear some of my readers reply: "Why, how can you ask such a question? You — are a republican." A republican! Yes, but that word specifies nothing. Res publico; that is, the public thing. Now, whoever is interested in public affairs — no matter under what form of government, may call himself a republican. Even kings are republicans. "Well, you are a democrat." No … "Then what are you?"”

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.

Country Living and Country Thinking, Preface, reported in Bartlett's Familiar Quotations, 10th ed. (1919).

Source: Discipline and Punish (1977), Chapter Three, The Gentle Way in Punishment

4 Burr. Part IV., 2379.
Dissenting in Millar v Taylor (1769)

Source: What is Property? (1840), Ch. I: "Method Pursued in this Work. The Idea of a Revolution"
Context: If I were asked to answer the following question: What is slavery? and I should answer in one word, It is murder, my meaning would be understood at once. No extended argument would be required to show that the power to take from a man his thought, his will, his personality, is a power of life and death; and that to enslave a man is to kill him. Why, then, to this other question: What is property! may I not likewise answer, It is robbery, without the certainty of being misunderstood; the second proposition being no other than a transformation of the first?
I undertake to discuss the vital principle of our government and our institutions, property: I am in my right. I may be mistaken in the conclusion which shall result from my investigations: I am in my right. I think best to place the last thought of my book first: still am I in my right.

“Why Nazism Was Socialism and Why Socialism Is Totalitarian,” lecture delivered at the Mises Institute’s “The Economics of Fascism: Supporters Summit 2005” in Auburn, Alabama (October 8, 2005) https://www.youtube.com/watch?v=XsaG-pJ_4RA&list=PLOCWSOHhjJPUQ9kkhBKV9js9tFJTPp3yC&index=3&t=0s

Original: (fr) La propriété exclusive est un vol dans la nature.

Quoted in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 615.

Source: Titus Groan (1946), Chapter 69 “Mr Rottcodd Again” (p. 393)

"Letter From Washington," http://www.panarchy.org/hess/libertarianism.html The Libertarian Forum 1, no. 6 http://web.archive.org/web/20071201123614/http://mises.org/journals/lf/1969/1969_06_15.pdf (15 June 1969), p. 2