 The Abhidharma’s ontological investigations occasioned a host of doctrinal problems that became the subject of an ongoing debate among the Buddhist schools. One primary controversy centered on the principle of impermanence: if all phenomena are impermanent, the Sautrāntika challenged the Sarvāstivāda and the Theravāda, then dharmas must be changing continuously and can neither exist in the past and future nor endure for any period of time, however short, in the present. On the other hand, the systematic analysis of experience in terms of momentary dharmas required the Abhidharma to provide a rigorous account of the processes that govern psychological and physical continuity. What fuels these processes is causal interaction, but the very notion of causation is allegedly compromised by the theory of momentariness. If causes, conditions, and their results are all momentary events, how can an event that has ended have a result? How can an event that undergoes distinct stages of origination, endurance, and cessation in a brief moment have causal efficacy? Notwithstanding their doctrinal differences, both the Sarvāstivāda and the Theravāda Abhidhamma had to confront these challenges, and they did so by formulating complex theories of immediate contiguity that grant causal efficacy.