Empiricism

The two problems resurrect the old difficulty of bridging the gap between appearance and reality. Seventeenth-century advocates of the new science joined Plato in sharply distinguishing the world of everyday experience from underlying reality. Empiricists, with their limited resources, have tended to stick close to the experiential surface of the world by either narrowing the gap between appearance and reality, denying the existence of an underlying reality altogether, adopting the skeptical position that we simply cannot know it, or rejecting all talk of a reality beyond experience as "metaphysical" and hence meaningless. In short, they have wavered over commitment to the reality of unobservable entities and processes.

Locke denied that we can know the real essences of things. Our classifications are not natural but artificial—conventions made for human convenience. Hume and the later positivists, with their verifiability theory of meaning, ruled out metaphysics as meaningless. Ernst Mach (1838–1916), the Viennese physicist and positivist, denied the existence of atoms and developed a phenomenalistic account of the world. Berkeley had denied the existence of matter with his principle, "To be is to be perceived or to perceive" (Principles of Human Knowledge, 1710). Only minds and ideas exist. Does the cat then go out of existence when it disappears beyond the sofa? No, because God (the biggest mind) still perceives it, replied Berkeley. Mill later used a logico-linguistic device to remove the need for God and thus obtain a full-fledged phenomenalism. In An Examination of Sir William Hamilton's Philosophy (1865), he attempted to reduce physical objects to "permanent possibilities of sensation," expressible by (impossibly) long series of statements about what a person would experience or would have experienced in such-and-such a situation. Russell, using the new symbolic logic to the same end, attempted to reduce mind itself to a logical construction out of experiences. He took the same line for the postulated theoretical entities of physics: "Wherever possible, logical constructions out of known objects are to be substituted for inferred entities" ("The Relation of Sense Data to Physics," in Mysticism and Logic, 1917). This was a halfway house between realism and instrumentalism or fictionalism. If electrons are logical constructions out of actual and possible laboratory operations and the resulting observations, then they are not real entities of underlying reality; but neither are they complete fictions. Rather, electron talk is a convenient, economical façon de parler.