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… is from page 352 of Karl Popper’s 1963 collection, Conjectures and Refutations: The Growth of Scientific Knowledge [2]; specifically, it’s from Popper’s 1956 essay (originally a 1954 address to the Mont Pelerin Society) entitled “Public Opinion and Liberal Principles”:
Truth is not manifest; and it is not easy to come by. The search for truth demands at least
(a) imagination
(b) trial and error
(c) the gradual discovery of our prejudices by way of (a) and (b), and of critical discussion.