It is no secret that various versions of logical empiricism argued for the importance of unified science. Carnap was a proponent of unity of science views, although he expressed this in different idioms at different times. In the Aufbau (1928) he spoke of the unity of the object domain secured through definability in the constitutional system, in his physicalist period he argued that a physicalist language could serve as the universal language of science, and in his mature philosophical work he investigated a variety of meanings that might be attached to reductionist projects for unifying various scientific fields. Our essay focuses less on Carnap’s several proposals than on a prior interpretive question: what was philosophically at stake for Carnap in the question of the unity of science? We begin with some suggestions in Carnap’s 1963 Intellectual Autobiography, where he calls unity of science “one of the main tenets of our general philosophical conception” and one in which Neurath’s emphasis on the “interdependence of all decisions … made a strong impression” on him. We follow Carnap in the suggestion that an account of his approach to unity of science does not take us into questions of unity of nature but rather of the scientific attitude and the unity of reason.

This chapter analyses the careers of two distinct narratives about Mach’s philosophical legacy that prevailed among German-speaking physicists and philosophers for more than a generation. Planck’s polemic against Mach sired the idea of a Machian philosophical system that was irreconcilable with modern physics, Boltzmann’s legacy foremost. But Planck also bereaved Mach’s positivism of its naturalist foundation and identified it straight with phenomenalism. In contrast, many Austrians considered the epistemologies of Mach and Boltzmann as even mutually supportive for a defence of empiricist indeterminism. Taking positivism in its original, more general understanding, they underscored Mach’s broader anti-metaphysical and empiricist stance, eventually adopting him as a standard-bearer for the new movement of Logical Empiricism. While these understandings were not necessarily tied to a positive or negative assessment, they often amounted to simplifications, transformations, or even contortions of Mach’s thinking, which made it increasingly difficult to declare oneself in Mach’s footsteps and simultaneously to advocate scientific modernism.