This article highlights the federal government’s role as a collector and arbiter of scientific knowledge of “the Indian,” in projects directed by Lewis Cass, Albert Gallatin, and Henry R. Schoolcraft; examines the linguistic precursor to biological essentialism; demonstrates white philologists’ reliance on Native tutors, some of whom also entered scientific and policy debates; and suggests why the federal government began moving toward English-only instruction even as biological notions of race gained ascendance. During the removal debates, Indian languages focused the attention of men of letters, statesmen, and the broader public. Peter S. Du Ponceau and Cass argued over the grammatical character of the “American languages,” with the former praising them and the latter attacking those tongues and the “philanthropic” philology. At stake was the future of Indian affairs and inquirers explored Native languages for evidence of Indians’ intellectual and moral capacity to be assimilated into U.S. society. In denying that language corresponded to social condition, Du Ponceau suggested that all Indians spoke according to a uniform, unchanging, and unique “plan of ideas.” He and other participants in the debate, such as Wilhelm von Humboldt and Schoolcraft, began to define, linguistically, a distinct and fixed “Indian mind.” Scholars of the early republic and antebellum era who wish to study scientific definitions of race must come to terms with linguistic ideas, which requires confronting the intercultural encounters, intellectual exchanges, and institutions through which they emerged.