the quotable Popper II

"We have seen that theories cannot be logically derived from observations. They can, however, clash with observations: they can contradict observations. This fact makes it possible to infer from observations that a theory is false. The possibility of refuting theories by observations is the basis of all empirical tests. For the test of the theory is, like every rigorous examination, always an attempt to show that the candidate is mistaken--that is, that the theory entails a false assertion. From a logical point of view, all empirical tests are therefore attempted refutations." (Conjectures and Refutations, p. 260)