Avicenna

Avicenna's central philosophical achievement was a work whose title undersells its scope. The Kitab al-Shifa, usually translated as the Book of Healing though it has nothing to do with medicine, is an encyclopedic system covering logic, physics, mathematics, and metaphysics, built to heal the soul's ignorance rather than the body's illness. In its metaphysical sections Avicenna developed a distinction that would occupy philosophers for centuries afterward, between a thing's essence, what it is, and its existence, that it is, arguing that for any ordinary thing these are separable and that existence must be added to essence from outside. From this he built an argument for a single being whose essence and existence cannot be separated, a Necessary Existent whose existence follows from its own nature and requires no external cause, functioning as the ultimate explanation for why anything contingent exists at all. This framework let Avicenna integrate Aristotelian causation and Neoplatonic emanation with the Islamic doctrine of a single necessary God, and the essence and existence distinction in particular crossed directly into Latin scholastic philosophy, where it shaped Thomas Aquinas's own arguments for God's existence. His shorter Kitab al-Najat, the Book of Salvation, offered a condensed version of the same system for readers without the stamina for the full encyclopedia.

Avicenna's philosophical output was produced under conditions almost none of it would suggest. Trained as a physician and reportedly self taught in philosophy by his own account, he entered the service of the Samanid court at Bukhara as a young man, gaining access to a celebrated royal library, but the Samanid state collapsed not long afterward, and for the rest of his life Avicenna moved between the courts of competing Persian rulers in Gurgan, Rayy, Hamadan, and Isfahan, serving at various points as physician, political vizier, and confidential advisor. That service was unstable, he was imprisoned for several months after backing the losing side in a succession dispute at Hamadan, and later fled in disguise to Isfahan to escape a ruler's hostility. He is reported to have dictated portions of his major works from memory while traveling or in hiding, and to have written with a speed and continuity his students found difficult to credit, given the political disorder surrounding him. He died in 1037 at Hamadan, reportedly from a digestive illness aggravated by the stress of accompanying his last patron on a military campaign. That an encyclopedic philosophical and medical system of the scope of the Book of Healing was produced under these conditions, rather than in any settled academic institution, remains one of the more striking facts of his career.