The reductive and positivist philosophy of science associated particularly with Bridgman. According to operationalism, propositions about theoretical entities such as particles are disguised propositions about the experiences resulting from definite scientific operations. Theoretical terms that are not so definable are to be avoided. Operationalism was supported by the overwhelming importance of Einstein’s recognition that we can mean no more by such a notion as absolute simultaneity than we can measure by signals of finite velocity. However, the view is equivalent to the radical empiricism that holds that theoretical entities are logical constructions from experience. It is now widely supposed that this is untenable, and that it is necessary to maintain some logical gap between theory and evidence.