Empiricism

===Logical empiricism===
{{Main|Logical positivism}}
Logical empiricism (also ''logical positivism'' or ''neopositivism'') was an early 20th-century attempt to synthesize the essential ideas of British empiricism (e.g. a strong emphasis on sensory experience as the basis for knowledge) with certain insights from [[mathematical logic]] that had been developed by [[Gottlob Frege]]  and [[Ludwig Wittgenstein]]. Some of the key figures in this movement were [[Otto Neurath]], [[Moritz Schlick]] and the rest of the [[Vienna Circle]], along with [[A. J. Ayer]], [[Rudolf Carnap]] and [[Hans Reichenbach]].

The neopositivists subscribed to a notion of philosophy as the conceptual clarification of the methods, insights and discoveries of the sciences. They saw in the logical symbolism elaborated by Frege (1848–1925) and [[Bertrand Russell]] (1872–1970) a powerful instrument that could rationally reconstruct all scientific discourse into an ideal, logically perfect, language that would be free of the ambiguities and deformations of natural language. This gave rise to what they saw as metaphysical pseudoproblems and other conceptual confusions. By combining Frege's thesis that all mathematical truths are logical with the early Wittgenstein's idea that all [[logical truth]]s are mere linguistic [[tautology (logic)|tautologies]], they arrived at a twofold classification of all propositions: the "analytic" (''a priori'') and the "synthetic" (''a posteriori'').<ref>[[Peter Achinstein|Achinstein, Peter]], and [[Stephen Francis Barker|Barker, Stephen F.]] (1969), The Legacy of Logical Positivism: Studies in the Philosophy of Science, Johns Hopkins University Press, Baltimore, MD.{{page needed|date=December 2013}}</ref> On this basis, they formulated a strong principle of demarcation between sentences that have sense and those that do not: the so-called "[[verification principle]]". Any sentence that is not purely logical, or is unverifiable, is devoid of meaning. As a result, most metaphysical, ethical, aesthetic and other traditional philosophical problems came to be considered pseudoproblems.<ref>Barone, Francesco (1986), Il neopositivismo logico, Laterza, Roma Bari.{{page needed|date=December 2013}}</ref>

In the extreme empiricism of the neopositivists—at least before the 1930s—any genuinely synthetic assertion must be reducible to an ultimate assertion (or set of ultimate assertions) that expresses direct observations or perceptions. In later years, Carnap and Neurath abandoned this sort of ''phenomenalism'' in favor of a rational reconstruction of knowledge into the language of an objective spatio-temporal physics.  That is, instead of translating sentences about physical objects into sense-data, such sentences were to be translated into so-called ''protocol sentences'', for example, "''X'' at location ''Y'' and at time ''T'' observes such and such".<ref>[[Nicholas Rescher|Rescher, Nicholas]] (1985), The Heritage of Logical Positivism, University Press of America, Lanham, MD.</ref>  The central theses of logical positivism (verificationism, the analytic–synthetic distinction, reductionism, etc.) came under sharp attack after World War II by thinkers such as [[Nelson Goodman]], [[Willard Van Orman Quine|W. V. Quine]], [[Hilary Putnam]], [[Karl Popper]], and [[Richard Rorty]]. By the late 1960s, it had become evident to most{{who|date=April 2021}} philosophers that the movement had pretty much run its course, though its influence is still significant among contemporary [[analytic philosophy|analytic philosophers]] such as [[Michael Dummett]] and other [[anti-realism|anti-realists]].