Karl Popper - Wikipedia

The history of science, like the history of all human ideas, is a history of irresponsible dreams, of obstinacy, and of error. But science is one of the very few human activities — perhaps the only one — in which errors are systematically criticized and fairly often, in time, corrected. This is why we can say that, in science, we often learn from our mistakes, and why we can speak clearly and sensibly about making progress there."

- Karl Popper in "Conjectures and Refutations: The Growth of Scientific Knowledge" (1963)

The game of science is, in principle, without end. He who decides one day that scientific statements do not call for any further test, and that they can be regarded as finally verified, retires from the game."

- Karl Popper in Ch. 2 "On the Problem of a Theory of Scientific Method", Section XI

In so far as a scientific statement speaks about reality, it must be falsifiable: and in so far as it is not falsifiable, it does not speak about reality."

— Karl Popper as quoted in "The Logic of Scientific Discovery" (2002)

Our aim as scientists is objective truth; more truth, more interesting truth, more intelligible truth. We cannot reasonably aim at certainty. Once we realize that human knowledge is fallible, we realize also that we can never be completely certain that we have not made a mistake."

- Karl Popper in "In Search of a Better World" (1994)

Always remember that it is impossible to speak in such a way that you cannot be misunderstood: there will always be some who misunderstand you."

- Karl Popper in "Unended Quest: An Intellectual Autobiography (1976)

Democracy and freedom do not guarantee the millennium. No, we do not choose political freedom because it promises us this or that. We choose it because it makes possible the only dignified form of human coexistence, the only form in which we can be fully responsible for ourselves. Whether we realize its possibilities depends on all kinds of things — and above all on ourselves."

- Karl Popper in "On Freedom" (1958)

It is wrong to think that belief in freedom always leads to victory; we must always be prepared for it to lead to defeat. If we choose freedom, then we must be prepared to perish along with it."

- Karl Popper in "On Freedom" (1958)

We must plan for freedom, and not only for security, if for no other reason than only freedom can make security more secure."

- Karl Popper in "The Open Society and Its Enemies" (1945) Vol 2, Ch. 21 "An Evaluation of the Prophecy"

It is often asserted that discussion is only possible between people who have a common language and accept common basic assumptions. I think that this is a mistake. All that is needed is a readiness to learn from one's partner in the discussion, which includes a genuine wish to understand what he intends to say. If this readiness is there, the discussion will be the more fruitful the more the partner's backgrounds differ."

- Karl Popper in "Conjectures and Refutations: The Growth of Scientific Knowledge" (1963)

The belief in a political Utopia is especially dangerous. This is possibly connected with the fact that the search for a better world, like the investigation of our environment, is (if I am correct) one of the oldest and most important of all the instincts."

- Karl Popper in "In Search of a Better World" (1994)

"There is an almost universal tendency, perhaps an inborn tendency, to suspect the good faith of a man who holds opinions that differ from our own opinions. … It obviously endangers the freedom and the objectivity of our discussion if we attack a person instead of attacking an opinion or, more precisely, a theory."

- Karl Popper in "The Importance of Critical Discussion"

"We all remember how many religious wars were fought for a religion of love and gentleness; how many bodies were burned alive with the genuinely kind intention of saving souls from the eternal fire of hell. Only if we give up our authoritarian attitude in the realm of opinion, only if we establish the attitude of give and take, of readiness to learn from other people, can we hope to control acts of violence inspired by piety and duty."

- Karl Popper in "Utopia and Violence" (1947)

What we should do, I suggest, is to give up the idea of ultimate sources of knowledge, and admit that all knowledge is human; that it is mixed with our errors, our prejudices, our dreams, and our hopes; that all we can do is to grope for truth even though it be beyond our reach. We may admit that our groping is often inspired, but we must be on our guard against the belief, however deeply felt, that our inspiration carries any authority, divine or otherwise. If we thus admit that there is no authority beyond the reach of criticism to be found within the whole province of our knowledge, however far it may have penetrated into the unknown, then we can retain, without danger, the idea that truth is beyond human authority. And we must retain it. For without this idea there can be no objective standards of inquiry; no criticism of our conjectures; no groping for the unknown; no quest for knowledge."

- Karl Popper on "Conjectures and Refutations: The Growth of Scientific Knowledge" (1963)

What a monument of human smallness is this idea of the philosopher king. What a contrast between it and the simplicity of humaneness of Socrates, who warned the statesmen against the danger of being dazzled by his own power, excellence, and wisdom, and who tried to teach him what matters most — that we are all frail human beings."

- Karl Popper in "The Open Society and Its Enemies" (1945)

Those who promise us paradise on earth never produced anything but a hell."

- Karl Popper, as quoted in "In Passing: Condolences and Complaints on Death, Dying, and Related Disappointments" (2005) by Jon Winokur, p. 144

http://en.wikipedia.org/wiki/Karl_popper
Sir Karl Raimund Popper, CH FRS FBA (28 July 1902 – 17 September 1994) was an Austro-British philosopher and a professor at the London School of Economics. He is regarded as one of the greatest philosophers of science of the 20th century; he also wrote extensively on social and political philosophy.
Popper is known for his attempt to repudiate the classical observationalist / inductivist form of scientific method in favour of empirical falsification. He is also known for his opposition to the classical justificationist account of knowledge which he replaced with critical rationalism, "the first non justificational philosophy of criticism in the history of philosophy" As well, he is known for his vigorous defense of liberal democracy and the principles of social criticism that he came to believe made a flourishing "open society" possible.