The idea of the fourth dimension of space has been of sustained interest to nineteenth-century and Modernist studies since the publication of Linda Dalrymple Henderson’s The Fourth Dimension and Non-Euclidean Geometry in Modern Art (1983). An idea from mathematics that was appropriated by occultist thought, it emerged in the fin de siècle as a staple of genre fiction and grew to become an informing idea for a number of important Modernist writers and artists. Describing the post-Euclidean intellectual landscape of the late nineteenth century, The Emergence of the Fourth Dimension works with the concepts derived from the mathematical possibilities of n-dimensional geometry—co-presence, bi-location, and interpenetration; the experiences of two consciousnesses sharing the same space, one consciousness being in two spaces, and objects and consciousness pervading each other—to examine how a crucially transformative idea in the cultural history of space was thought and to consider the forms in which such thought was anchored. It identifies a corpus of higher-dimensional fictions by Conrad and Ford, H.G. Wells, Henry James, H.P. Lovecraft, and others and reads these closely to understand how fin de siècle and early twentieth-century literature shaped and were in turn shaped by the reconfiguration of imaginative space occasioned by the n-dimensional turn. In so doing it traces the intellectual history of higher-dimensional thought into diverse terrains, describing spiritualist experiments and how an extended abstract space functioned as an analogue for global space in occult groupings such as the Theosophical Society.