ABSTRACT. Traditional attempts to delineate the distinctive rationality of modern science have taken it for granted that the purpose of empirical research is to test judgments. The choice of concepts to use in those judgments is therefore seen either a matter of indifference (Popper) or as important choice which must be made, so to speak, in advance of all empirical research (Carnap). I argue that scientific method aims precisely at empirical testing of concepts, and that even the simplest scientific experiment or observation results in conceptual change.