In his critical philosophy Kant treats space, in varying contexts, as an intuition, as a concept, and as an idea of reason. Space as an intuition and space as a concept are familiar enough to readers of the Critique of Pure Reason, though the relation between them is not so easy to make out. Space — more precisely, absolute space — as an idea of reason, occurring as it does only in the Metaphysical Foundations of Natural Science, is perhaps not so well known. To set the stage for my discussion of Kant's absolute space I shall first try to locate it with respect to his other two spaces; after describing the markedly relativistic character of Kant's idea of absolute space, I shall then explain how Kant attempts to reconcile this idea with the reality of circular motion.