Logical positivism was a philosophical movement that began in the 1920s in the Vienna Circle. The philosopher A.J. Ayer (1910-1989) was instrumental in popularizing logical positivism in the English-speaking world. His most famous and systematic exposition and defense of logical positivism was given in his book Language, Truth and Logic. At the heart of logical positivism is the principle of verification (also known as the verification principle, verifiability criterion, principle of verifiability, the verification criterion of meaning, etc.). The principle of verification is a criterion for determining whether or not a sentence is meaningful, where a sentence is meaningful just in case it succeeds in expressing a proposition. A major motivation of the principle is to enable us to distinguish between meaningful statements of logic and natural science and (according to logical positivists) meaningless statements of metaphysics, ethics, theology, and aesthetics. The principle tries to achieve this goal by dividing all propositions into two categories: (1) analytic propositions and (2) empirically verifiable propositions.
