The twelfth-century Persian philosopher Suhrawardi was the key figure in the transition of Islamic philosophy from the neo-Aristotelianism of Avicenna to the mystically oriented Islamic philosophy of later centuries. Suhrawardi's "Illuminationist" philosophy was a vigorous reassertion of Neoplatonism at a time when Sufism was becoming a major presence in Islamic thought and society. This book traces the intellectual background of Suhrawardi's thought and of the Greek roots of non-Aristotelian philosophy in the Islamic world. Suhrawardi placed himself in an intellectual tradition that sprang From the "Ancients, " the philosophical and mystical tradition of Hermes Trismegistus and his successors in both Greece and the Orient. The author argues that Suhrawardi typifies an approach to philosophy characteristic of Neoplatonism, in which Pythagoras is the key pre-Socratic, Plato is the central figure in the history of philosophy, Aristotle is respected but corrected by reference to Pythagoras and Plato, and philosophy is ultimately an eclectic revelation known symbolically by different nations. Mystical intuition is a key philosophical tool and symbolism is of particular importance. The Leaven of the Ancients provides a translation of Suhrawadi's famous dream, in which Aristotle reveals the epistemological foundations of Suhrawardi's Illuminationist system. The book also analyzes the role played by Suhrawardi and his approach to philosophy in turning Islamic civilization away from physical science toward a subtle mystical psychology.