Popper heard Einstein lecture on his new theory of relativity: “I remember only that I was dazed. This thing was quite beyond my understanding.” But he was struck by Einstein’s willingness to subject his theory to empirical tests that might disprove it: Thus I arrived, by the end of 1919, at the conclusion that the scientific attitude was the critical attitude, which did not look for verifications but for crucial tests; tests which could refute the theory tested, though they could never establish it. In that single italicized word germinated Popper’s greatest and most influential idea.
