<P> Vandervert (1980 / 1988) described in scientific detail a simple, everyday illustration of an operational definition in terms of making a cake (i.e., its recipe is an operational definition used in a specialized laboratory known as the household kitchen). Similarly, the saying, if it walks like a duck and quacks like a duck, it must be some kind of duck, may be regarded as involving a sort of measurement process or set of tests (see duck test). </P> <P> Despite the controversial philosophical origins of the concept, particularly its close association with logical positivism, operational definitions have undisputed practical applications . This is especially so in the social and medical sciences, where operational definitions of key terms are used to preserve the unambiguous empirical testability of hypothesis and theory . Operational definitions are also important in the physical sciences . </P> <P> The Stanford Encyclopedia of Philosophy entry on scientific realism, written by Richard Boyd, indicates that the modern concept owes its origin in part to Percy Williams Bridgman, who felt that the expression of scientific concepts was often abstract and unclear . Inspired by Ernst Mach, in 1914 Bridgman attempted to redefine unobservable entities concretely in terms of the physical and mental operations used to measure them . Accordingly, the definition of each unobservable entity was uniquely identified with the instrumentation used to define it . From the beginning objections were raised to this approach, in large part around the inflexibility . As Boyd notes, "In actual, and apparently reliable, scientific practice, changes in the instrumentation associated with theoretical terms are routine, and apparently crucial to the progress of science . According to a' pure' operationalist conception, these sorts of modifications would not be methodologically acceptable, since each definition must be considered to identify a unique' object' (or class of objects)." However, this rejection of operationalism as a general project destined ultimately to define all experiential phenomena uniquely did not mean that operational definitions ceased to have any practical use or that they could not be applied in particular cases . </P> <P> The special theory of relativity can be viewed as the introduction of operational definitions for simultaneity of events and of distance, that is, as providing the operations needed to define these terms . </P>

Who gave behaviorism the notion of operational definitions