Can God be proven by reason alone?

Most arguments for God start from the world: its beginning, its order, its dependence. The ontological argument starts from the idea of God alone. In the 11th century Anselm of Canterbury defined God as “that than which nothing greater can be conceived” and argued that such a being must exist in reality, since a being that existed only in the mind would not be the greatest conceivable.

The paper traces the argument and its critics across nearly a thousand years. Gaunilo, a monk of Anselm's own time, replied that the same reasoning could prove a perfect island. Aquinas objected that people do not all share Anselm's concept of God. Descartes restated the argument: existence belongs to a supremely perfect being as surely as angles adding up to 180 degrees belong to a triangle. Kant's reply, that existence is not a property added to a thing's description, was long taken as decisive. In 1974 Alvin Plantinga revived the argument using the logic of possible worlds: if it is even possible that a maximally great being exists, then it exists necessarily. Critics answer that one can just as easily assume it is possible that no such being exists, and reach the opposite conclusion.

The paper also turns to Islamic philosophy. Ibn Sina (Avicenna) argued from contingent things, things that could have not existed, to a Necessary Existent (wajib al-wujud), and the paper notes that this proof became a cornerstone of later Islamic thought. Mulla Sadra, a 17th-century Persian philosopher, argued from the nature of existence itself, treating it as one reality in degrees with God at its summit. The authors explain that this rests on ideas close to wahdat al-wujud, the “unity of existence”, which Sunni scholars such as Ibn Taymiyyah and Ahmad Sirhindi opposed for blurring the line between Creator and creation, so Sadra's proof was not accepted in Sunni theology.

The authors conclude that no ontological argument is a knock-down proof, since each depends on a premise a sceptic can reject. Its lasting value, they argue, lies in clarifying what God would be: not one being among others, but a necessary being than whom none is greater.