Can a religion grounded in revelation make room for philosophy that follows reason wherever it leads? The question sounds modern, but it was argued with great skill in the Islamic world a thousand years ago.

Two kinds of inquiry

Two traditions grew side by side. Kalam was rational theology: arguing about God, free will and revelation with reason, starting from the Qur'an. Its practitioners included the Mu'tazila, who stressed God's justice and human free will, and later the Ash'aris, who stressed God's absolute power. Falsafa, from the Greek word philosophia, was philosophy in the Greek manner, working mainly from Aristotle and Neoplatonist writers. The interest here is in what thinkers writing in Arabic did with those Greek ideas.

Al-Kindi and al-Farabi

Al-Kindi (about 800 to about 870), from a noble Arab family and active in Baghdad, is known as the philosopher of the Arabs. Al-Kindi argued that truth should be welcomed whatever its source, even from foreign peoples, and that a seeker of truth should not be ashamed to learn from those who came before. He tried to show that philosophy and Islam agree, for example by arguing that the world had a beginning in time, as revelation taught.

Al-Farabi (about 870 to 950), who worked in Baghdad and later in Syria, was called the Second Teacher, second only to Aristotle, for his work on logic. In his book on the virtuous city, he blended Plato's Republic with Islamic ideas, describing an ideal state led by a ruler who is both philosopher and prophet.

Ibn Sina's philosophy

Ibn Sina, whom we met as a physician, was also the most influential philosopher of the Islamic world. He distinguished between what a thing is, its essence, and the fact that it exists, and argued that everything depends for its existence on something else, except one being that exists necessarily by its own nature, which he identified with God. He also proposed a famous thought experiment, the "floating man." A person created in midair, unable to see and touching nothing, would still know that he exists, which Ibn Sina took to show that the self is aware of itself apart from the body.

Al-Ghazali's challenge

Abu Hamid al-Ghazali (1058 to 1111) was a star professor at the Nizamiyya college in Baghdad when, in 1095, he suffered a personal crisis, left his post and spent years as a wandering Sufi. In The Incoherence of the Philosophers he turned the philosophers' own logic against them. He accepted their mathematics and logic, but he condemned three of their teachings as unbelief: that the world is eternal, that God knows only universals and not particular things, and that there is no bodily resurrection. He also challenged the idea of necessary causes. When cotton touches fire and burns, he argued, we observe only that the burning follows the contact; we never observe the fire making it happen. God creates the burning, and could in principle withhold it. The Scottish philosopher David Hume would make a related point about observing cause and effect seven centuries later, though for very different purposes.

Ibn Rushd's reply

In twelfth-century Cordoba, the judge and physician Ibn Rushd (1126 to 1198), known in Latin as Averroes, answered al-Ghazali point by point in The Incoherence of the Incoherence. In a shorter work, the Decisive Treatise, he argued that Islamic law actually requires those capable of it to study philosophy, since the Qur'an calls on people to reflect on creation, and that demonstrated truth cannot contradict revealed truth; where the two seem to clash, scripture should be read in a non-literal way. He also wrote detailed commentaries on nearly all of Aristotle's works, which made him famous in Latin Europe.

Did al-Ghazali kill philosophy?

An old story claims that al-Ghazali's attack ended philosophy and science in the Islamic world. Most historians now reject it. Philosophy continued, especially in Iran, where Ibn Sina's system was studied and developed for centuries; logic became a standard part of religious education, helped by al-Ghazali's own endorsement of it; and some of the most original astronomy came after his death. What did change was the standing of falsafa as a rival authority to religious scholarship: its ideas were increasingly absorbed into theology rather than pursued as a separate path.

Worked example: the cotton and the fire

A student asks whether al-Ghazali's view of causation makes science pointless: if God could stop fire from burning, why study fire? Al-Ghazali's answer was that God's habit is reliable, so we can expect fire to burn and act on that expectation, while recognizing that the regularity depends on God rather than on a necessity inside things. Ibn Rushd replied that denying real causes destroys the very idea of knowledge, since to know a thing is to know its causes. A modern scientist might side with neither, studying regularities and testing them again and again without claiming either divine habit or built-in necessity. Working through the exchange shows that a debate about theology was also a debate about what scientific knowledge is, a question still very much alive.