Historiographers of linguistics have frequently pointed out the presence of the Humboldtian term ‘inner form’ in Franz Boas’ (1858–1942) work on linguistic categorization and have suggested a link to Heymann Steinthal’s (1823–1899) Völkerpsychologie and psycholinguistics. This essay demonstrates, however, that Boas’ discourse on the inner form of language, grammatical categories, and the human mind did not develop in a unilinear fashion from the work of Steinthal. Although Boas adhered to a Steinthalian notion of inner form of language and linguistic relativism and his research on Native American languages was initially guided by Steinthal’s criteria ‘form’ and ‘material,’ Boas’ texts also exhibit some disontinuities with Steinthal’s work, and they carry traces linking his linguistics to the work of Adolf Bastian (1826–1905), Theodor Waitz (1826–1864), Herbert Spencer (1820–1903), Wilhelm Wundt (1832–1920), and Daniel Garrison Brinton (1837–1899). Boas strategically distanced his discourse from the hierarchical thinking underlying the work of Steinthal, Spencer, and Wundt. As part of this distancing strategy, Boas shifted from Herbartian psychology, informing his early phonetic theory, to an associationist framework, and he postulated a universal mental faculty of abstraction as a necessary condition for human language to arise. Boas also introduced the concept of ‘coordinate elements’ in morphology, and he assumed the existence of universal relational functions in the languages of the world.