Karl Popper, answered from the texts and cited to the page.
Critical rationalism begins with a question I found more tractable than the traditional one. Instead of asking "What are the sources of our knowledge?" — a question that presupposes some pure, authoritative origin — I ask: "How can we hope to detect and eliminate error?" The shift matters enormously.1 The traditional question sends philosophers hunting for foundations, for certainty, for an untainted spring from which knowledge flows.
My question accepts from the outset that no such spring exists. Xenophanes saw this clearly, two and a half millennia ago. His verses are worth taking seriously: the gods did not reveal all things from the beginning; through seeking, men find what is better. And as for certain truth — no man has known it, nor will he know it; all is but a woven web of guesses.2
That is not scepticism in the paralysing sense. It is the honest starting point. The answer to my question — how do we detect and eliminate error? — is: by criticizing the theories or guesses of others, and, if we can train ourselves to do so, by criticizing our own.3 This is what I call critical rationalism: a view, an attitude, a tradition we owe to the Greeks, very different from the Cartesian search for indubitable foundations and different again from Kant's theoretical philosophy, though Kant approached something like it in ethics through his principle of autonomy, which insists that we must judge for ourselves whether any command — even a divine one — is moral or immoral to obey.4
The critical or rationalist tradition, so understood, was invented only once, in Ionia, and lost after two or three centuries, perhaps because the Aristotelian doctrine of epistēmē — certain, demonstrable knowledge — drove out the earlier, more honest recognition that we only guess.5 It was revived by Galileo, and incorporated into modern science, though many scientists still labour under the Baconian myth that knowledge grows by induction from observation.6
Here is what that myth gets wrong. Observations and experiments play an indispensable role in science, but not the role inductivists assign them. Their significance depends entirely on whether they can be used to criticize theories.7 They are critical arguments, not generative sources. We do not read theories off the face of nature; we propose bold conjectures and then subject them to the most severe criticism we can devise.
There is, I believe, only one element of rationality in our attempts to know the world: the critical examination of our theories.8 The theories themselves are guesswork. If you ask me how I know something, my honest answer is that I do not know — I only propose a guess, and I am most happy when you criticize it and offer counter-proposals, which I will in turn try to criticize.9
One further point, which I regard as central. The critical rationalist is aware — as Parmenides was not — that he can never prove his theories, but can at best refute some of their competitors. He therefore never tries to establish a theory about the world; he does not believe in foundations.10 Yet he may believe — as I do — that if we produce many competing ideas and criticize them severely, we may, if we are lucky, get nearer to the truth.
That is the method of conjectures and refutations: taking many risks, producing many competing hypotheses, making many mistakes, and trying to correct or eliminate some of those mistakes by criticism.11
My modified question, 'How can we hope to detect error?' may be said to derive from the view that such pure, untainted and certain sources do not exist, and that questions of origin or of purity should not be confounded with questions of validity, or of truth.Conjectures and Refutations, p. 30
But as for certain truth, no man has known it, Nor will he know it; neither of the gods, Nor yet of all the things of which I speak. And even if by chance he were to utter The final truth, he would himself not know it; For all is but a woven web of guesses.Conjectures and Refutations, p. 30
The proper answer to my question 'How can we hope to detect and eliminate error?' is, I believe, 'By criticizing the theories or guesses of others and--if we can train ourselves to do so--by criticizing our own theories or guesses.'Conjectures and Refutations, p. 30
This answer sums up a position which I propose to call 'critical rationalism'... in the field of ethics, of moral knowledge, it was approached by Kant with his principle of autonomy. This principle expresses his realization that we must not accept the command of an authority, however exalted, as the basis of ethics.Conjectures and Refutations, p. 30
To my knowledge the critical or rationalist tradition was invented only once. It was lost after two or three centuries, perhaps owing to the rise of the Aristotelian doctrine of epistēmē, of certain and demonstrable knowledge.Conjectures and Refutations, pp. 151–152
It was rediscovered and consciously revived by the Renaissance, especially by Galileo Galilei.Conjectures and Refutations, pp. 151–152
the significance of observations and experiments depends entirely upon the question whether or not they may be used to criticize theories.Conjectures and Refutations, pp. 151–152
There is only one element of rationality in our attempts to know the world: it is the critical examination of our theories.Conjectures and Refutations, pp. 151–152
If you ask me, 'How do you know?' my reply would be, 'I don't; I only propose a guess. If you are interested in my problem, I shall be most happy if you criticize my guess, and if you offer counter-proposals, I in turn will try to criticize them.'Conjectures and Refutations, pp. 151–152
the critical rationalist is (unlike Parmenides) aware that he can never prove his theories but can, at best, refute some of their competitors. Thus the critical rationalist never tries to establish a theory about the world: he does not believe in 'foundations'.The World of Parmenides, p. 172
This method is the method of conjectures and refutations; it is the method of taking many risks, by producing many (competing) hypotheses; of making many mistakes; and of trying to correct or eliminate some of these mistakes by aThe World of Parmenides, p. 172