Campbell Norman Robert (1880–1949), British physicist and philosopher of science. A successful experimental physicist, Campbell (with A. Wood) discovered the radioactivity of potassium. His analysis of science depended on a sharp distinction between experimental laws and theories. Experimental laws are generalizations established by observations. A theory has the following structure. First, it requires a (largely arbitrary) hypothesis, which in itself is untestable. To render it testable, the theory requires a ‘dictionary’ of propositions linking the hypothesis to scientific laws, which can be established experimentally. But theories are not merely logical relations between hypotheses and experimental laws; they also require concrete analogies or models. Indeed, the models suggest the nature of the propositions in the dictionary. The analogies are essential components of the theory, and, for Campbell, are nearly always mechanical. His theory of science greatly influenced Nagel’s The Structure of Science (1961). See also PHILOSOPHY OF SCIENCE , THEORETICAL TER. R.E.B.