Karl Popper - The Logic of Scientific Discovery (1934)

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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
I am inclined to think that scientific discovery is impossible without faith in ideas which are of a purely speculative kind, and sometimes even quite hazy; a faith which is completely unwarranted from the point of view of science...

Karl Popper
Science is not a system of certain, or well-established statements, nor is it a system which steadily advances towards a state of finality.. Like Bacon we might describe our own contemporary science... as consisting of "anticipations, rash and premature," and as "prejudices."

Karl Popper
But some of these theories are so bold that they can clash with reality: they are the testable theories of science. And when they clash, then we know that there is a reality; something that can inform us that our ideas are mistaken.

Karl Popper
Is the world ruled by strict laws or not? This question I regard as metaphysical. The laws we find are always hypotheses; which means that they may always be superseded, and that they may possibly be deduced from probability estimates. Yet denying causality would be the same as attempting to persuade the theorist to give up his search; and that such an attempt cannot be backed by anything like a proof...

Karl Popper
Science never pursues the illusory aim of making its answers final or even probable. Its advance is rather toward an infinite yet attainable aim: that of ever discovering new, deeper, and more general problems.

Karl Popper
Whenever we propose a solution to a problem, we ought to try as hard as we can to overthrow our solution, rather than defend it.

Karl Popper
Now this principle of induction cannot be a purely logical truth like a tautology or an analytic statement....

Karl Popper
What is to be called "science" and who is to be called a "scientist" must always remain a matter of convention or decision.

Karl Popper
Science does not aim, primarily, at high probabilities. It aims at a high informative content, well backed by experience. But a hypothesis may be very probable simply because it tells us nothing, or very little.

Karl Popper
It is not intuitive ease I am after, but rather a point of view which is sufficiently definite to clear up some difficulties, and to be criticized in rational terms. (Bohr's complementarity cannot be so criticized, I fear; it can only be accepted or denounced - perhaps as being ad hoc, or as being irrational, or as being hopelessly vague.)

Karl Popper
The old scientific ideal of episteme - of absolutely certain, demonstrable knowledge - has proved to be an idol. The demand for scientific objectivity makes it inevitable that every scientific statement must remain tentative for ever.

Karl Popper
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
I shall certainly admit a system as empirical or scientific only if it is capable of being tested by experience. These considerations suggest that not the verifiability but the falsifiability of a system is to be taken as a criterion of demarcation…It must be possible for an empirical scientific system to be refuted by experience.

Karl Popper
There can be no ultimate statements in science: there can be no statements in science which can not be tested, and therefore none which cannot in principle be refuted, by falsifying some of the conclusions which can be deduced from them.

Karl Popper
... no matter how many instances of white swans we may have observed, this does not justify the conclusion that all swans are white.

Karl Popper
Bold ideas, unjustified anticipations, and speculative thought, are our only means for interpreting nature: our only organon, our only instrument, for grasping her. And we must hazard them to win our prize. Those among us who are unwilling to expose their ideas to the hazard of refutation do not take part in the scientific game.

Karl Popper
I think that we shall have to get accustomed to the idea that we must not look upon science as a 'body of knowledge,' but rather as a system of hypotheses; that is to say, as a system of guesses or anticipations which in principle cannot be justified, but with which we work as long as they stand up to tests, and of which we are never justified in saying that we know they are 'true' or 'more or less certain' or even 'probable.'

Karl Popper