“It is easier to criticize than to correct our past errors.”

Source: Conjectures and Refutations: The Growth of Scientific Knowledge (1963), Ch. 1 "Science : Conjectures and Refutations"
Context: 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.

Le cose mal fatte e di gran tempo passate son più agevoli a riprendere che ad emendare.
Second Day, Fifth Story
The Decameron (c. 1350)

Source: Women Can't Hear What Men Don't Say (2000), p. 41.

"On Revolutionary Morality" (1958)
1950's, On Revolutionary Morality (1958)

Academy of Achievement interview (1991)
Context: The idea of being constructive, creative, positive, in trying to bring out the best in one's own self and the best in others follows from what I've just been saying. Again, I repeat my belief in us, in ourselves, as the product of the process of evolution, and part of the process itself. I think of evolution as an error-making and error-correcting process, and we are constantly learning from experience. It's the need to dedicate one's self in that way, to one's own self, and to choose an activity or life that is of value not only to yourself but to others as well.

"On Freedom" in All Life is Problem Solving (1999)
Context: When I speak of reason or rationalism, all I mean is the conviction that we can learn through criticism of our mistakes and errors, especially through criticism by others, and eventually also through self-criticism. 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. The emphasis here is on the idea of criticism or, to be more precise, critical discussion. The genuine rationalist does not think that he or anyone else is in possession of the truth; nor does he think that mere criticism as such helps us achieve new ideas. But he does think that, in the sphere of ideas, only critical discussion can help us sort the wheat from the chaff. He is well aware that acceptance or rejection of an idea is never a purely rational matter; but he thinks that only critical discussion can give us the maturity to see an idea from more and more sides and to make a correct judgement of it.