Julian Willard in The Philosophers' Magazine:
“A towering figure” is how W. V. O. Quine, himself one of the greatest twentieth century philosophers, described Rudolf Carnap (1891-1970): “I see him as the dominant figure in philosophy from the 1930’s onward, as Russell had been in the decades before.”
A German-born philosopher who moved to America in 1935, Carnap was one of the leaders of the Vienna Circle, a movement commonly referred to as Logical Positivism. Its members – including Moritz Schlick, Otto Neurath, Herbert Feigl, and Hans Hahn – aimed to solve particular problems in the philosophy of mathematics and the physical and social sciences. Carnap himself made important contributions to semantics, the philosophy of science, probability, and inductive logic.
A central creed of logical positivism, in part inspired by Wittgenstein, was the verification principle – that sentences gain their meaning by specification of the means by which we determine their truth or falsity. The Circle’s manifesto, which Carnap completed with Neurath and Hahn, articulated a philosophical standpoint which was to reverberate around English-speaking universities for a generation: “If anyone asserts, ‘There is a God’, ‘The primary cause of the world is the Unconscious’…we do not say ‘What you say is false’; rather, we ask him ‘What do you mean by your statements?’” And since these assertions are neither testable against experience, nor analytic – somehow true by definition – it follows that they are meaningless.