In the philosophy of science, the attempt to define all scientific concepts in terms of specifically described operations of measurement and observation. The length of a rod, for example, may be defined as the number of times a certain stick can be laid end to end alongside it. Propositions that are not amenable to verification through measurement and observation are rejected as meaningless (see alsological positivism). Operationalists rejected the idea of nature as a thing-in-itself existing behind the appearances observed in experimentation. Operationalism is closely associated with the work of the U.S. physicist Percy W. Bridgman (1882–1961).