“Those who won our independence by revolution were not cowards. They did not fear political change. They did not exalt order at the cost of liberty. To courageous, self-reliant men, with confidence in the power of free and fearless reasoning applied through the processes of popular government, no danger flowing from speech can be deemed clear and present, unless the incidence of the evil apprehended is so imminent that it may befall before there is opportunity for full discussion. If there be time to expose through discussion the falsehood and fallacies, to avert the evil by the processes of education, the remedy to be applied is more speech, not enforced silence.”

Charles Evans Hughes, De Jonge v. Oregon, 299 U.S. 353, 365 (1937).
Judicial opinions
Context: Freedom of speech and of the press are fundamental rights which are safeguarded by the due process clause of the Fourteenth Amendment of the Federal Constitution. [... ] The right of peaceable assembly is a right cognate to those of free speech and free press, and is equally fundamental. As this Court said in United States v. Cruikshank, 92 U. S. 542, 552: The very idea of a government, republican in form, implies a right on the part of its citizens to meet peaceably for consultation in respect to public affairs and to petition for a redress of grievances. The First Amendment of the Federal Constitution expressly guarantees that right against abridgment by Congress. But explicit mention there does not argue exclusion elsewhere. For the right is one that cannot be denied without violating those fundamental principles of liberty and justice which lie at the base of all civil and political institutions — principles which the Fourteenth Amendment embodies in the general terms of its due process clause. [... ] These rights may be abused by using speech or press or assembly in order to incite to violence and crime. The people, through their legislatures may protect themselves against that abuse. But the legislative intervention, can find constitutional justification only by dealing with the abuse. The rights themselves must not be curtailed. The greater the importance of safeguarding the community from incitements to the overthrow of our institutions by force and violence, the more imperative is the need to preserve inviolate the constitutional rights of free speech, free press and free assembly in order to maintain the opportunity for free political discussion, to the end that government may be responsive to the will of the people and that changes, if desired, may be obtained by peaceful means. Therein lies the security of the Republic, the very foundation of constitutional government.

Source: Something Happened

Notes for an oration at Braintree (Spring 1772)
1770s

Speech http://hansard.millbanksystems.com/commons/1854/mar/31/war-with-russia-the-queens-message in the House of Commons on the debate on war with Russia (31 March 1854).
1850s

Plato, Phaedo

As quoted by George H. W. Bush in remarks while presenting National Medals of Science and Technology http://bushlibrary.tamu.edu/research/papers/1990/90111300.html (13 November 1990). This might be a paraphrase of statements from his introduction to "Science The Endless Frontier" (1945), rather than a direct quote. (see below)

"A University's Bequest to Youth" Speech in Toronto (10 October 1936)
Canadian Occasions (1940)

Jeff Cooper's Commentaries, vol. 4, no. 16, December, 1996

1961, Address to ANPA
Context: Today no war has been declared — and however fierce the struggle may be, it may never be declared in the traditional fashion. Our way of life is under attack. Those who make themselves our enemy are advancing around the globe. The survival of our friends is in danger. And yet no war has been declared, no borders have been crossed by marching troops, no missiles have been fired.
If the press is awaiting a declaration of war before it imposes the self-discipline of combat conditions, then I can only say that no war ever posed a greater threat to our security. If you are awaiting a finding of "clear and present danger," then I can only say that the danger has never been more clear and its presence has never been more imminent.
It requires a change in outlook, a change in tactics, a change in missions — by the government, by the people, by every businessman or labor leader, and by every newspaper. For we are opposed around the world by a monolithic and ruthless conspiracy that relies primarily on covert means for expanding its sphere of influence — on infiltration instead of invasion, on subversion instead of elections, on intimidation instead of free choice, on guerrillas by night instead of armies by day. It is a system which has conscripted vast human and material resources into the building of a tightly knit, highly efficient machine that combines military, diplomatic, intelligence, economic, scientific and political operations. Its preparations are concealed, not published. Its mistakes are buried, not headlined. Its dissenters are silenced not praised. No expenditure is questioned, no rumor is printed, no secret is revealed. It conducts the Cold War, in short, with a war-time discipline no democracy would ever hope or wish to match.
Nevertheless, every democracy recognizes the necessary restraints of national security — and the question remains whether those restraints need to be more strictly observed if we are to oppose this kind of attack as well as outright invasion.