"Operationalism" is a program that aims at linking all scientific concepts to experimental procedures and at cleansing science of operationally undefinable terms, which it regards as being devoid of empirical meaning. Scientists adopted the operational approach to their subject before the principles of operationalism were made articulate. Operationalist theory was erected not on the basis of independent philosophical considerations but upon what was already implicit in the working practice of scientists. P. W. Bridgman, the Nobel Prize–winning physicist who is commonly regarded as the founder of operationalism, emphasized this point when he said, "it must be remembered that the operational point of view suggested itself from the observation of physicists in action" ("The Present State of Operationalism," in The Validation of Scientific Theories, edited by Philipp Frank, Boston, 1956, p. 79).

A fairly nontechnical illustration of the kinds of development in science in which one can discern an implicit...