According to Oliver Leaman, Mulla Sadra is arguably the single most important and influential philosopher in the Muslim world in the last four hundred years.[1][2] Mulla Sadra brought "a new philosophical insight in dealing with the nature of reality" and created "a major transition from essentialism to existentialism" in Islamic philosophy,[3] although his existentialism is not to be readily compared to Western existentialism. His was a question of existentialist cosmology as it pertained to Allah, and thus differs considerably from the individual, moral, and/or social, questions at the heart of Russian, French, German, or American Existentialism. Mulla Sadra's philosophy synthesized Avicennism, Shahab al-Din Suhrawardi's Illuminationist philosophy, Ibn Arabi's Sufi metaphysics, and the theology of the Ash'ari school and Twelvers.[4]