KARL POPPER

Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.

Our knowledge can only be finite, while our ignorance must necessarily be infinite.

We have become makers of our fate when we have ceased to pose as its prophets.

Piecemeal social engineering resembles physical engineering in regarding the ends as beyond the province of technology.

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.

It is impossible to speak in such a way that you cannot be misunderstood.

Science may be described as the art of systematic over-simplification.

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

There is no history of mankind, there are only many histories of all kinds of aspects of human life. And one of these is the history of political power. This is elevated into the history of the world.

No rational argument will have a rational effect on a man who does not want to adopt a rational attitude.

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

Good tests kill flawed theories; we remain alive to guess again.

All things living are in search of a better world.

Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them… We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law, and we should consider incitement to intolerance and persecution as criminal, in the same way as we should consider incitement to murder, or to kidnapping, or to the revival of the slave trade, as criminal.

Only freedom can make security secure.

How can we so organize political institutions that bad or incompetent rulers can be prevented from doing too much damage?

It is not his possession of knowledge, of irrefutable truth, that makes the man of science, but his persistent and recklessly critical quest for truth.

Science must begin with myths and with the criticism of myth.

A Marxist could not open a newspaper without finding on every page confirming evidence for his interpretation of history; not only in the news, but also in its presentation — which revealed the class bias of the paper — and especially of course what the paper did not say.

The criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.

Optimism is a duty. The future is open. It is not predetermined. No one can predict it, except by chance. We all contribute to determining it by what we do. We are all equally responsible for its success.

… utopian social engineering

You can choose whatever name you like for the two types of government. I personally call the type of government which can be removed without violence democracy, and the other tyranny.

Reason like science, grows by way of mutual criticism; the only possible way of planning its growth is to develop those institutions that safeguard. the freedom of thought

Science does not rest upon solid bedrock. The bold structure of its theories rises, as it were, above a swamp. It is like a building erected on piles. The piles are driven down from above into the swamp, but not down to any natural or 'given' base; and if we stop driving the piles deeper, it is not because we have reached firm ground. We simply stop when we are satisfied that the piles are firm enough to carry the structure, at least for the time being.