 The philosophical discussion that developed around the members of the Vienna Circle was associated with the formulation of logical empiricism and the banner of ‘unity of science against metaphysics’, but it cannot be adequately understood without taking into account the diversity of evolving projects it accommodated. Carnap (much influenced by the neo-Kantian tradition, Einstein’s theories of relativity, Husserl’s phenomenology, Frege’s logic and, i.e., Hilbertian axiomatic approach to mathematics as well as Russell’s own logicism and his philosophical project of logical constructions) was concerned with the conditions of the possibility of objective knowledge, which he considered especially manifest in the formal exactness of scientific claims (Carnap 1928). Schlick (inspired by turn-of-the-century French conventionalists such as Rey, Duhem and Poincaré, Wittgenstein’s philosophy of representation of facts, as well as Einstein’s relativity theories and Hilbert’s implicit definitions and axiomatic approach to mathematics) was interested in the meaning of terms in which actual scientific knowledge of reality is expressed, as well as its foundation on true and certain beliefs about reality (Schlick 1918, 1934). Neurath (influenced by Mach, varieties of French conventionalism, especially Duhem’s, and Marxism) sought to explore the empirical and historical conditions of scientific practice in both the natural and social sciences (Neurath 1931/1973, 1932a/1983). Outside the Vienna Circle, Popper (influenced by Hume, Kant and Austrian child psychology) was interested in addressing the logical and normative issues of justification and demarcation of objective and empirical scientific knowledge, without relying on descriptive discussions of the psychology of subjective experience and meaning (Popper 1935/1951). 