Avicenna (Ibn Sina)

Ibn Sina was born near Bukhara, son of an official of the Samanid dynasty. A prodigy, he had memorised the Qur'an at ten, practised medicine at sixteen and claimed at eighteen to have mastered all the sciences. He said he read Aristotle's Metaphysics forty times without understanding it, until a chance purchase of a short treatise by al-Farabi made it clear. Curing the Samanid emir won him access to the royal library, and he read everything.

After the Samanids fell he spent his life moving between courts in Central Asia and Persia, serving as vizier, sitting in prison, writing at night and governing by day, and notorious for wine and pleasure. On the road he wrote The Cure, an encyclopaedia of logic, physics, mathematics and metaphysics, and The Canon of Medicine, used as a textbook in European medical schools into the seventeenth century.

The heart of his metaphysics is the distinction between necessary and possible existence. The essence of any thing does not by itself include existence; its existence needs a cause, and the chain must end in a being whose essence is existence, the Necessary Existent, God. That argument and the essence–existence distinction reached Aquinas through Latin translation. His 'flying man' thought experiment holds that a person suspended in air with no sensation would still be aware of existing, so the soul is not the body.