  The distinction between essence and existence can be traced back as far as Boethius in the fifth century. It was later developed by Islamic thinkers such as Avicenna. But the issue did not become a major philosophical problem until it was taken up by Aquinas in the thirteenth century. The issue arose not as part of an effort to establish God’s existence on a priori grounds (as mentioned above, Aquinas was one of the staunchest critics of the ontological argument), but out of concern to distinguish God from finite spiritual entities such as angels. Like many scholastic philosophers, Aquinas believed that God is perfectly simple and that created beings, in contrast, have a composite character that accounts for their finitude and imperfection. Earthly creatures are composites of matter and form (the doctrine of hylomorphism), but since purely spiritual beings are immaterial, Aquinas located their composite character in the distinction between essence and existence.