The Book of Healing and a New Kind of Necessity

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.