“I am so completely convinced of the importance of this cause, as it is related to the solution of a problem no patriotic citizen should neglect, that I look upon every attempt to stimulate popular interest and activity in its behalf as a duty of citizenship.”

Source: The Conscience of a Conservative (1960), p. 15
Context: I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution, or that have failed their purpose, or that impose on the people an unwarranted financial burden. I will not attempt to discover whether legislation is "needed" before I have first determined whether it is constitutionally permissible. And if I should later be attacked for neglecting my constituents' "interests," I shall reply that I was informed that their main interest is liberty and that in that cause I am doing the very best I can.

"The Jury System" (February 1890)
Context: In this, our land, we are called upon to give but little in return for the advantages which we receive. Shall we give that little grudgingly? Our definition of patriotism is often too narrow. Shall the lover of his country measure his loyalty only by his service as a soldier? No! Patriotism calls for the faithful and conscientious performance of all of the duties of citizenship, in small matters as well as great, at home as well as upon the tented field.

As quoted in The World of Mathematics (1956) Edited by J. R. Newman

Statement to a reporter in the Boston Record, 14 April 1903. (quoted in Alpheus Thomas Mason, Brandeis: A Free Man's Life (1946), p. 122.)
Commonly paraphrased as "The most important office is that of the private citizen" or "The most important political office is that of the private citizen", and sometimes misattributed to his dissenting opinion in Olmstead v. United States.
Extra-judicial writings

Arrow, Kenneth J., and Gerard Debreu. " Existence of an equilibrium for a competitive economy http://cowles.econ.yale.edu/P/cp/p00b/p0087.pdf." Econometrica: Journal of the Econometric Society (1954): p. 265

Source: 1950s-1960s, "Existence of an equilibrium for a competitive economy." 1954, p. 265

Edward Abbey, "A patriot must always be ready to defend his country against his government." as written in "A Voice Crying in the Wilderness" (Vox Clamantis en Deserto): Notes from a Secret Journal (1990), ISBN 0312064888.
Misattributed

Waldersee, quoted in the Le Petit Parisien newspaper, c. 1888

Letter to Giovanni Boccaccio (28 April 1373) as quoted in Petrarch : The First Modern Scholar and Man of Letters (1898) edited by James Harvey Robinson and Henry Winchester Rolfe, p. 417
Context: I certainly will not reject the praise you bestow upon me for having stimulated in many instances, not only in Italy but perhaps beyond its confines also, the pursuit of studies such as ours, which have suffered neglect for so many centuries; I am, indeed, almost the oldest of those among us who are engaged in the cultivation of these subjects. But I cannot accept the conclusion you draw from this, namely, that I should give place to younger minds, and, interrupting the plan of work on which I am engaged, give others an opportunity to write something, if they will, and not seem longer to desire to reserve everything for my own pen. How radically do our opinions differ, although, at bottom, our object is the same! I seem to you to have written everything, or at least a great deal, while to myself I appear to have produced almost nothing.