This article examines the ontology and politics of Peter Sloterdijk's Spheres trilogy, focusing in particular upon the notion of microspherical enclosure explicated in the first volume of this series. Noting Sloterdijk's unusual alignment of his philosophy with media theory, three main contentions are put forward. Firstly, that Sloterdijk's reconfiguration of Heidegger's fundamental ontology represents a largely unacknowledged renunciation of the primacy of Being-towards-death in the authentic existence of Dasein, foregrounding instead an originary co-belonging between mother and child. Secondly, that Sloterdijk borrows from media theory a concern regarding the facticity of all communication, grounding philosophical discourse in the determinate locality of its origin, but does so while exalting a pre-natal communicative immediacy that would seem to disparage the everydayness of Dasein. Finally, that Sloterdijk's oft-justified scepticism regarding globalization often retreats into an anti-cosmopolitanism that, in its nostalgia for the comfort, security and immediacy of the matrixial co-belonging, evinces a covert but potentially noxious politics of exclusion.