Karl Popper

The growth of our knowledge is the result of a process closely resembling what Darwin called 'natural selection'; that is, the natural selection of hypotheses: our knowledge consists, at every moment, of those hypotheses which have shown their (comparative) fitness by surviving so far in their struggle for existence, a competitive struggle which eliminates those hypotheses which are unfit.

If you can't say it simply and clearly, keep quiet, and keep working on it till you can.

No particular theory may ever be regarded as absolutely certain.... No scientific theory is sacrosanct.

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.

A rationalist is simply someone for whom it is more important to learn than to be proved right; someone who is willing to learn from others - not by simply taking over another's opinions, but by gladly allowing others to criticize his ideas and by gladly criticizing the ideas of others

Thus science must begin with myths, and with the criticism of myths; neither with the collection of observations, nor with the invention of experiments, but with the critical discussion of myths, and of magical techniques and practices.

I am opposed to looking upon logic as a kind of game. ... One might think that it is a matter of choice or convention which logic one adopts. I disagree with this view.

To give a causal explanation of an event means to deduce a statement which describes it, using as premises of the deduction one or more universal laws, together with certain singular statements, the initial conditions ... We have thus two different kinds of statement, both of which are necessary ingredients of a complete causal explanation.

The influence (for good or ill) of Plato's work is immeasurable. Western thought, one might say, has been Platonic or anti-Platonic, but hardly ever non-Platonic.

The Conspiracy Theory of Society... [is] a typical result of the secularization of a religious superstition. The belief in the Homeric gods whose conspiracies explain the history of the Trojan War is gone. The gods are abandoned. But their place is filled by powerful men or groups - sinister pressure groups whose wickedness is responsible for all the evils we suffer from - such as the Learned Elders of Zion, or the monopolists, or the capitalists, or the imperialists.

The future is always present, as a promise, a lure and a temptation.

I see now more clearly than ever before that even our greatest troubles spring from something that is [as] admirable and sound as it is dangerous – from our impatience to better the lot of our fellows.

I remained a socialist for several years, even after my rejection of Marxism; and if there could be such a thing as socialism combined with individual liberty, I would be a socialist still. For nothing could be better than living a modest, simple, and free life in an egalitarian society.

When we enter a new situation in life and are confronted by a new person, we bring with us the prejudices of the past and our previous experiences of people. These prejudices we project upon the new person. Indeed, getting to know a person is largely a matter of withdrawing projections; of dispelling the smoke screen of what we imagine he is like and replacing it with the reality of what he is actually like.

A theory which is not refutable by any conceivable event is non-scientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.

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

Propose theories which can be criticized. Think about possible decisive falsifying experiments-crucial experiments. But do not give up your theories too easily-not, at any rate, before you have critically examined your criticism.

Psychologism is, I believe, correct only in so far as it insists upon what may be called 'methodological individualism' as opposed to 'methodological collectivism'; it rightly insists that the 'behaviour' and the 'actions' of collectives, such as states or social groups, must be reduced to the behaviour and to the actions of human individuals. But the belief that the choice of such an individualist method implies the choice of a psychological method is mistaken.

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

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.

I have come to the conclusion that Darwinism is not a testable scientific theory, but a metaphysical research programme.