The idea of retrocausality is also found in Indian philosophy. It was defended by at least two Indian Buddhist philosophers, Prajñākaragupta (ca. 8th–9th century) and Jitāri (ca. 940–1000), the latter wrote a specific treatise on the topic, the Treatise on Future Cause (Bhāvikāraṇavāda). In the 1950s, Michael Dummett wrote in opposition to such definitions, stating that there was no philosophical objection to effects preceding their causes. This argument was rebutted by fellow philosopher Antony Flew and, later, by Max Black. Black's "bilking argument" held that retrocausality is impossible because the observer of an effect could act to prevent its future cause from ever occurring. A more complex discussion of how free will relates to the issues Black raised is summarized by Newcomb's paradox. Essentialist philosophers have proposed other theories, such as the existence of "genuine causal powers in nature" or by raising concerns about the role of induction in theories of causality.