Instrumentalism and Operationalism are the methodological doctrines associated respectively with Milton Friedman and Paul Samuelson. Each has a long philosophical history. Instrumentalism was the 18th-century doctrine created to deal with the Newton mechanics; Operationalism was the early 20th-century doctrine created to deal with Einstein’ general relativity. With Instrumentalism one can say that theories do not have to be true, just useful – as Friedman argued in 1953. With Operationalism one is required to express theories only in terms of observable and measurable variables. Samuelson's early work was designed to demonstrate how theory can be made operational and thus potentially refutable.