In the course of his critique of the Buddhist doctrine of universal momentariness, Udayana argues for an isomorphism between our understandings of space and time, which is meant to undercut the Buddhists’ well-known “inference from existence.” The present paper examines these arguments from Udayana’s Ātmatattvaviveka, together with Ratnakīrti’s treatment of them in his Kṣaṇabhaṅgasiddhi Anvayātmikā. As an historical study, the paper aims to elucidate the connections between Udayana and Ratnakīrti, and the implications of those connections for the dependence of the (...) inference from existence upon various arguments which appear elsewhere in Ratnakīrti’s corpus. As a work of philosophical interpretation, the paper will clarify what is at stake in the local debate over the space–time isomorphism. Ratnakīrti’s position will best be understood as an account on which different simple causal properties are ascribed, or indexed, to an allegedly persisting entity at different times, while Udayana will prefer an account on which complex properties indexed to the place and/or time of the effect—for instance, “generating a sprout in this particular place” or “producing a visual awareness at a certain time”—will belong to a persisting thing throughout its entire existence. Furthermore, the acceptance or rejection of space and time as substantial entities in their own right, as distinct from the entities which are conventionally said to exist in space and time, will have important implications for the accounts of causality that each thinker can accept. (shrink)