Until the end of the 19th century, architectural theory was based firmly on practice, belonging primarily to the domain of architects concerned with construction, materiality, and design. A significant turning point came with the work of art historian August Schmarsow, who was the first to establish space as an autonomous category within architectural theory. He argued that the perception of space by the human subject is a fundamental precondition to architectural activity, and that the design of space constitutes the essence of architecture. This intervention initiated a reorientation in the understanding of the built environment that has remained effective into the modern era. This study investigates the conditions, sources, and stages of development that shaped this conceptual shift. First comprehensive analysis of Schmarsow’s writing on architectural theoryExamination of intellectual- and scientific-historical contexts as well as interdisciplinary referencesReconstructing an overall picture of Schmarsow’s architectural theory