Medieval Islam's great lawyer, theologian, and mystic, al-Ghazali is known to Muslims as hujjat al-Islam, "the proof of Islam." The appellation refers to his role in defending "orthodox" Islam and to his refutation of doctrines he regarded as contrary to Islamic teaching. His refutations include Tahafut al-Falasifah (The Incoherence of the Philosophers, circa 1094), an incisive critique of Islamic philosophy, particularly that of Avicenna. This work, which had repercussions in medieval and Renaissance European thought, questioned basic tenets of Islamic Aristotelianism, including the concept of necessary causal connection. It also leveled a devastating logical criticism on Islamic Neoplatonic emanative philosophy as developed by Avicenna. At the same time, al-Ghazali admired Avicenna's logic, which he regarded as a philosophically neutral tool of knowledge. He wrote several excellent expositions of it, recommending it to his fellow theologians and lawyers. His writings also include important works on Islamic law. Al-Ghazali, however, is...