The greatest and most influential voice of the Ash'arites was the medieval theologian Abu Hamid al-Ghazali (also known as Algazel; died 1111). In his bookThe Incoherence of the Philosophers, al-Ghazali vigorously attacked philosophy and philosophers — both the Greek philosophers themselves and their followers in the Muslim world (such as al-Farabi and Avicenna). Al-Ghazali was worried that when people become favorably influenced by philosophical arguments, they will also come to trust the philosophers on matters of religion, thus making Muslims less pious. Reason, because it teaches us to discover, question, and innovate, was the enemy; al-Ghazali argued that in assuming necessity in nature, philosophy was incompatible with Islamic teaching, which recognizes that nature is entirely subject to God's will: "Nothing in nature," he wrote, "can act spontaneously and apart from God." While al-Ghazali did defend logic, he did so only to the extent that it could be used to ask theological questions and wielded as a tool to undermine philosophy. Sunnis embraced al-Ghazali as the winner of the debate with the Hellenistic rationalists, and opposition to philosophy gradually ossified, even to the extent that independent inquiry became a tainted enterprise, sometimes to the point of criminality. It is an exaggeration to say, as Steven Weinberg claimed in the Times of London, that after al-Ghazali "there was no more science worth mentioning in Islamic countries"; in some places, especially Central Asia, Arabic work in science continued for some time, and philosophy was still studied somewhat under Shi'ite rule. (In the Sunni world, philosophy turned into mysticism.) But the fact is, Arab contributions to science became increasingly sporadic as the anti-rationalism sank in.
The Ash'ari view has endured to this day. Its most extreme form can be seen in some sects of Islamists. For example, Mohammed Yusuf, the late leader of a group called the Nigerian Taliban, explained why "Western education is a sin" by explaining its view on rain: "We believe it is a creation of God rather than an evaporation caused by the sun that condenses and becomes rain." The Ash'ari view is also evident when Islamic leaders attribute natural disasters to God's vengeance, as they did when they said that the 2010 eruption of Iceland's Eyjafjallajökull volcano was the result of God's anger at immodestly dressed women in Europe. Such inferences sound crazy to Western ears, but given their frequency in the Muslim world, they must sound at least a little less crazy to Muslims. As Robert R. Reilly argues inThe Closing of the Muslim Mind (2010), "the fatal disconnect between the creator and the mind of his creature is the source of Sunni Islam's most profound woes."