 Karl Popper’s  philosophy of science has been influential among economists, as among other scientists. Popper defends what he calls a falsificationist methodology (1968, 1969). Scientists should formulate theories that are “logically falsifiable” — that is, inconsistent with some possible observation reports. “All crows are black” is logically falsifiable; it is inconsistent with (and would be falsified by) an observation report of a red crow. (Probabilistic claims are obviously not in this sense falsifiable.) Popper insists on falsifiability on the grounds that unfalsifiable claims that rule out no observations are uninformative. They provide no guidance concerning what to expect, and there is nothing to be learned from testing them. Second, Popper maintains that scientists should subject theories to harsh test and should be willing to reject them when they fail the tests. Third, scientists should regard theories as at best interesting conjectures. Passing a test does not confirm a theory or provide scientists with reason to believe it. It only justifies on the one hand continuing to employ the hypothesis (since it has not yet been falsified) and, on the other hand, devoting increased efforts to attempting to falsify it (since it has thus far survived testing). Popper has defended what he calls “situational logic” (which is basically rational choice theory) as the correct method for the social sciences (1967, 1976). There appear to be serious tensions between Popper’s falsificationism and his defense of situational logic, and his discussion of situational logic has not been as influential as his falsificationism. For discussion of how situational logic applies to economics, see Hands (1985a).