Frontier: the border between two countries; the limits of civilization; the bounds of established knowledge; a new field of activity. At a time when all frontiers (borders, boundaries, margins, limits) are being—often violently—challenged, erased or reinforced, it might be a matter of urgency to take up and rethink the very concept of frontier itself. But is there even such a concept, to be found or constructed? That is what this book begins to cast into doubt, on the basis of a reading of Kant, for whom the frontier turns out to be both the very element of his thought and the permanent frustration of his conceptuality. Following what Kant himself would call this “guiding thread,” first in the “political” writings and then in the still little-read “Critique of teleological judgment,” I try to bring out a complex, abyssal, fractal structure, which always leaves a residue of nature—violence—in every frontier (including conceptual frontiers), and which complicates Kant’s most explicit, most rational arguments (which always tend towards cosmopolitanism and so-called perpetual peace) by adding to them an element of reticence or interruption. As it turns out that there can be perpetual peace only in death, we must interrupt the teleological movement that always might take us there, we must maintain some frontiers (and therefore a certain violence) in the very place where everything led us to believe that we should hope for their pacific disappearance, if only in the infinite perspective of the Idea of Reason. Neither critique of Kant nor return to Kant, this book also proposes a new reflection on philosophical reading, for which thinking the frontier is both an essential resource and the recurrent, fruitful, interruption.