Avicenna (Ibn Sīnā)

Avicenna was a polymath who memorized the Quran by ten, mastered Aristotle by eighteen, served as court physician and vizier, and somehow found time to write the Book of Healing (an encyclopedia of all known philosophy and science) and the Canon of Medicine (the standard medical textbook in both the Islamic world and Europe for six hundred years).

His central philosophical move was the distinction between essence and existence. The essence of a unicorn is well defined; whether unicorns exist is a separate question. Existence is something added to essence — and only in God do they coincide, because God is that whose essence is to exist.

This distinction will be picked up by Aquinas and become a load-bearing pillar of medieval metaphysics. Without Avicenna there is no scholastic philosophy. Through him and his successor Averroes, the Islamic world preserved and transformed Greek philosophy while medieval Europe was still in its Latin shell.