“Books and ideas are the most effective weapons against intolerance and ignorance.”

What Does God Want Us to Do About Russia? (1948)
Context: It is impossible to use an atomic bomb here to defend the population of our own city. Likewise we cannot use our bacterial weapons here in our own country. These weapons of annihilation must be used offensively. And against their use no effective defense is possible.

RTNDA Convention Speech (1958)
Context: This instrument can teach, it can illuminate; yes, and even it can inspire. But it can do so only to the extent that humans are determined to use it to those ends. Otherwise it's nothing but wires and lights in a box. There is a great and perhaps decisive battle to be fought against ignorance, intolerance and indifference. This weapon of television could be useful.
Stonewall Jackson, who knew something about the use of weapons, is reported to have said, "When war comes, you must draw the sword and throw away the scabbard." The trouble with television is that it is rusting in the scabbard during a battle for survival.

Part 4, Section 7
A Treatise of Human Nature (1739-40), Book 1: Of the understanding
Context: This deficiency in our ideas is not, indeed, perceived in common life, nor are we sensible, that in the most usual conjunctions of cause and effect we are as ignorant of the ultimate principle, which binds them together, as in the most unusual and extraordinary. But this proceeds merely from an illusion of the imagination; and the question is, how far we ought to yield to these illusions. This question is very difficult, and reduces us to a very dangerous dilemma, whichever way we answer it. For if we assent to every trivial suggestion of the fancy; beside that these suggestions are often contrary to each other; they lead us into such errors, absurdities, and obscurities, that we must at last become asham'd of our credulity. Nothing is more dangerous to reason than the flights of the imagination, and nothing has been the occasion of more mistakes among philosophers. Men of bright fancies may in this respect be compar'd to those angels, whom the scripture represents as covering their eyes with their wings. This has already appear'd in so many instances, that we may spare ourselves the trouble of enlarging upon it any farther.

Essays on Education quoted in The New York Times (24 February 1959).
Variant ending: The source of better ideas is freedom. The surest path to wisdom is liberal education.
Context: Books won't stay banned. They won't burn. Ideas won't go to jail. In the long run of history, the censor and the inquisitor have always lost. The only sure weapon against bad ideas is better ideas. The source of better ideas is wisdom. The surest path to wisdom is a liberal education.

Also quoted in "Warrior for Peace" by David Talbot, in TIME (2 July 2007), p. 50 http://www.time.com/time/specials/2007/article/0,28804,1635958_1635999_1634954-6,00.html
1961, Address at the University of Washington
Context: We increase our arms at a heavy cost, primarily to make certain that we will not have to use them. We must face up to the chance of war, if we are to maintain the peace. We must work with certain countries lacking in freedom in order to strengthen the cause of freedom. We find some who call themselves neutral who are our friends and sympathetic to us, and others who call themselves neutral who are unremittingly hostile to us. And as the most powerful defender of freedom on earth, we find ourselves unable to escape the responsibilities of freedom, and yet unable to exercise it without restraints imposed by the very freedoms we seek to protect. We cannot, as a free nation, compete with our adversaries in tactics of terror, assassination, false promises, counterfeit mobs and crises. We cannot, under the scrutiny of a free press and public, tell different stories to different audiences, foreign and domestic, friendly and hostile. We cannot abandon the slow processes of consulting with our allies to match the swift expediencies of those who merely dictate to their satellites. We can neither abandon nor control the international organization in which we now cast less than 1 percent of the vote in the General Assembly. We possess weapons of tremendous power — but they are least effective in combating the weapons most often used by freedom's foes: subversion, infiltration, guerrilla warfare, civil disorder. We send arms to other peoples — just as we send them the ideals of democracy in which we believe — but we cannot send them the will to use those arms or to abide by those ideals. And while we believe not only in the force of arms but in the force of right and reason, we have learned that reason does not always appeal to unreasonable men — that it is not always true that "a soft answer turneth away wrath" — and that right does not always make might. In short, we must face problems which do not lend themselves to easy or quick or permanent solutions. And we must face the fact that the United States is neither omnipotent or omniscient — that we are only 6 percent of the world's population — that we cannot impose our will upon the other 94 percent of mankind — that we cannot right every wrong or reverse each adversity — and that therefore there cannot be an American solution to every world problem.

Tactics and Strategy of the Latin American Revolution (1962)

1960s

The Drowned and the Saved (1986)
Context: In countries and epochs in which communication is impeded, soon all other liberties wither; discussion dies by inanition, ignorance of the opinion of others becomes rampant, imposed opinions triumph. The well-known example of this is the crazy genetics preached in the USSR by Lysenko, which in the absence of discussion (his opponents were exiled to Siberia) compromised the harvests for twenty years. Intolerance is inclined to censor, and censorship promotes ignorance of the arguments of others and thus intolerance itself: a rigid, vicious circle that is hard to break.