“The mystery is how a conception that is vulnerable to such obvious counterexamples survived for so long. I can explain it only by a weakness of the scholarly mind that I have often observed in myself. I call it theory-induced blindness: Once you have accepted a theory, it is extraordinarily difficult to notice its flaws. As the psychologist Daniel Gilbert has observed, disbelieving is hard work.”

Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 13

Source: A Brief History of Time (1988), Ch. 1
Context: Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.

Letter to John Scott, quoted in More Letters of Charles Darwin: A Record of His Work in a Series of Hitherto Unpublished Letters http://books.google.com/books?id=8FXwAAAAMAAJ&, volume 2, page 323 (1903)
Other letters, notebooks, journal articles, recollected statements

From his autobiography My Young Years (1973), quoted in Carol Krucoff (August 13, 1982) "FOCUS: With a Little Bit of Good Luck", The Washington Post, p. D5.

Objecting to the placing of observables at the heart of the new quantum mechanics, during Heisenberg's 1926 lecture at Berlin; related by Heisenberg, quoted in Unification of Fundamental Forces (1990) by Abdus Salam ISBN 0521371406
1920s

New Pathways in Science: Messenger Lectures 1934 (1947), p. 211.

Source: Sir William Herschel: His Life and Works (1880), Ch.4 "Life and Works" quote from his paper "Nature and Construction of the Sun and Fixed Stars" (1795).
Context: I should not wonder if, considering all this, we were induced to think that nothing remained to be added; and yet we are still very ignorant in regard to the internal construction of the sun.... The spots have been supposed to be solid bodies, the smoke of volcanoes, the scum floating on an ocean of fluid matter, clouds, opaque masses, and to be many other things.... The sun itself has been called a globe of fire, though, perhaps, metaphorically.... It is time now to profit by the observations we are in possession of. I have availed myself of the labors of preceding astronomers, but have been induced thereto by my own actual observation of the solar phenomena.<!-- p. 145-146

Source: The reality of the Mass Media (2000), p. 117.

Source: Just a Theory: Exploring the Nature of Science (2005), Chapter 2, “Just a Theory: What Scientists Do” (p. 24)