In 1922, the physicist and philosopher Moritz Schlick was appointed professor of the philosophy of inductive science at the University of Vienna. Three years later he organized a Thursday evening discussion group of philosophically-minded mathematicians and scientists. Though its membership varied over time, the group met regularly for the next eleven years, and through the efforts of its members a new philosophy was born. The philosophy became known as logical positivism and the group took for itself the label, the Vienna Circle. Some of the more significant members over the years included Rudolf Carnap, Herbert Feigl, Phillip Frank, Kurt Gödel, Hans Hahn, Karl Menger (the economist’s son), Otto Neurath, and Friedrich Waismann.
Though the logical positivists were confident that their new analyses constituted ‘an altogether decisive turning point in philosophy’, 1 they also acknowledged that many earlier thinkers influenced their work. Those mentioned included most of the European philosophers in the empiricist tradition, anyone who made a contribution to symbolic logic or axiomatics, and finally, any thinker who showed anti-metaphysical or anti-speculative tendencies in his work. 2 Three philosophers, however, stand out as having had a truly significant influence on the development of logical positivism: Ernst Mach, Bertrand Russell, and Ludwig Wittgenstein. Mach’s theory of elements, which proposed that all phenomena (even psychical) could be reduced to complexes of sensations, and his dismissal of the idea of a ‘thing-in-itself’ behind one’s sensations of an object as metaphysics, laid a strong positivist foundation upon which the logical empiricists could build. Bertrand Russell’s pathbreaking efforts (in conjunction with Alfred North Whitehead in