SummaryFranz Boas’ (1858–1942) Statements on phonetics can only be appreciated adequately if they are read against the background of 19th-century experimental psychology, acoustics, physiology, and psychophysics. This paper demonstrates that Boas adhered to a theory of phonetics which included a physical and a psychological component. The former component was informed by contemporary ideas on phonetics put forward by Hermann Helmholtz (1821–1894), Wilhelm Wundt (1832–1920), Hermann Paul (1846–1921), and Mikołaj Kruszewski (1851–1887). Within this component, Boas included the production of speech sounds, their acoustical nature, and the mechanical workings of the ear. For Boas, speech-sounds were averages consisting of groups of oscillations which gave each sound its peculiar character. The ear analyzed speech-sounds into their component groups of oscillations, and the resulting sensations were individually transmitted into consciousness. The psychological component of Boas’ theory was influenced by Gustav Fechner’s (1801–1887) psychophysics, and it was initially based on Herbartian psychology. This second component included mental representations (Vorstellungen) of sounds, the process of apperception, and Fechner’s law of thresholds (Schwellengesetz). Boas’ theory presupposed a model of the mind as machine in which the ear was seen as a mechanical extension of the mind. Within this mechanical model of the mind, the recognition of speech-sounds was deterministically governed by the law of thresholds and the process of apperception. The interaction of the law of thresholds with the process of apperception was responsible for the phenomenon of alternating sounds. With the help of his theory, Boas countered positions which considered such seemingly fluctuating sounds as the hallmark of ‘primitive’ languages. In order to distance himself from Heymann Steinthal’s (1823–1899) Eurocentric linguistics, which was rooted in the Herbartian tradition, Boas later abandoned his Herbartian framework in favor of an associationist theory of psychology.