Definition
The ontological argument is an a priori argument for the existence of God — one that proceeds from concepts and definitions alone, without appeal to observation of the world. First formulated by the eleventh-century philosopher Anselm of Canterbury, it tries to show that God's existence follows from the very idea of God.
Anselm defines God as "that than which nothing greater can be conceived." He then argues that a being existing in reality is greater than one existing only in the mind; so if the greatest conceivable being existed only as an idea, an even greater being — one that also existed in reality — could be conceived. To avoid contradiction, the greatest conceivable being must exist in reality.
Why it matters
How it works
The argument's force lies in its claim that denying God's existence is, once the definition is accepted, a kind of contradiction. Modern versions recast it using modal logic, arguing that if God's existence is even possible, then a necessary being exists in every possible world.
The most influential criticism, advanced by Immanuel Kant, is that "existence" is not a predicate — it adds nothing to the content of a concept, so one cannot define a thing into being. An early critic, the monk Gaunilo, made a similar point by parodying the argument to "prove" the existence of a perfect island. Dawkins regards the argument as a verbal trick rather than a real demonstration, while acknowledging that philosophers continue to take its modern forms seriously.