Throughout history, divides between the sciences and the humanities have been drawn in many different ways. This essay shows that the notion of a divide became more urgent and pronounced in the second half of the nineteenth century. While this shift has several causes, the essay focuses on the rise of the social sciences, which is interpreted as posing a profound challenge to the established disciplines of the study of humankind. This is demonstrated by zooming in on linguistics, one of the key traditional disciplines of the humanities. Through the assumption of a correspondence between mental and linguistic categories, psychology became of central importance in the various conceptions of linguistics that emerged in the nineteenth century. Both linguistics and psychology were very much engaged in a process of discipline formation, and opinions about the proper directions of the fields varied considerably. Debates on these issues catalyzed the construction of more radical divisions between the sciences and the humanities. Both Wilhelm Dilthey’s dichotomy between understanding and explanation and Wilhelm Windelband’s dichotomy between nomothetic and idiographic sciences respond to these debates. While their constructions are often lumped together, the essay shows that they actually meant very different things and have to be treated accordingly.