Moritz Schlick, was founder and leader of the German WIENER KREIS, a group of philosophers, scientists, and mathematicians was formed in the 1920s that met regularly in Vienna to investigate scientific language and scientific methodology. The philosophical movement associated with the Circle has been called variously logical positivism, logical empiricism, scientific empiricism, neopositivism, and the unity of science movement. The work of its members, although not unanimous in the treatment of many issues, was distinguished, first, by its attention to the form of scientific theories, in the belief that the logical structure of any particular scientific theory could be specified quite apart from its content. Second, they formulated a verifiability principle or criterion of meaning, a claim that the meaningfulness of a proposition is grounded in experience and observation. For this reason, the statements of ethics, metaphysics, religion, and aesthetics were held to be assertorically meaningless. Third, and as a result of the two other points, a doctrine of unified science was espoused. Thus, no fundamental differences were seen to exist between the physical and the biological sciences or between the natural and the social sciences. Moritz Schlick was an epistemologist and philosopher of science. Among the members of the Wiener Kreis were Gustav Bergmann, Rudolf Carnap, Herbert Feigl, Philipp Frank, Kurt G鐰el, Otto Neurath, and Friedrich Waismann; and among the members of a cognate group, the Gesellschaft f empirische Philosophie ("Society for Empirical Philosophy"), which met in Berlin, were Carl Hempel and Hans Reichenbach. A formal declaration of the group's intentions was issued in 1929 with the publication of the manifesto Wissenschaftliche Weltauffassung: Der Wiener Kreis ("Scientific Conception of the World: The Vienna Circle"), and in that year the first in a series of congresses organized by the group took place in Prague.