Ontology is a branch of philosophy dealing with the nature of being. Questions posed by ontology include what kind of entities exist and how they might relate or be hierarchically structured.
What we call the ontological argument sounds rather daunting. But it is just a theological position that tries to prove the existence of the greatest being of all, namely God.
Several ontological arguments can be found. The most famous was devised by St. Anselm of Canterbury.
St. Anselm describes God in his Proslogion II as “aliquid quo nihil majus cogitari possit” (that than which nothing greater can be conceived). For Anselm, such a being cannot merely live in the “imagination” or “understanding” but must fully exist.¹ Because the greatest conceivable being must exist in all of reality and not just in the mind, God is the greatest conceivable being which by necessity exists.
St. Thomas Aquinas rejected this argument on rational grounds, although Aquinas being a cornerstone of Catholic theology did believe in God.
For those unfamiliar with philosophy and theology, this happens quite often. One can believe in something but find shortcomings in a particular argument for its existence or truthfulness.²
The philosopher René Descartes forwarded an outlook similar to Anselm’s. Descartes begins with a method of doubt.³ After coming to the conclusion, “Je pense, donc je suis” (I think, therefore I am), his next question is: “how do I know that the outside world truly exists?”4
Thomas Leahey notes that Descartes was not the first to look at things this way.
St. Augustine [354–430 CE] had said, “If I am deceived, I exist,” and Parmenides [515-445 BCE] had said, “For it is the same thing to think and to be.”5
Descartes’ answer to the question of whether or not the outside world exists involves God.
For Descartes, God exists by necessity. God must exist to be perfect. A perfect God also by necessity is good. And a good God would not deceive his creatures into believing in an outside world if such a thing did not exist.
But not only that. Descartes believed that his reasoning about the existence of a good God necessarily originated from beyond himself, like some kind of small revelation.6
² For example, I believe in the efficacy of the Eucharist but do not agree that its benefits arise solely from the fact that the sacrament is a social gathering. For me, spiritual elements must be included in an explanation.
³ Some see this as a sham, saying Descartes believed all along. A similar critique arises with Plato who, some contend, pretends through Socrates to start asking questions from scratch when really he is guiding his argument toward foregone conclusions—that is, the doctrine of the Forms.
4 This is similar to solipsism.
5 Link broken since last revision. 😦
6 Leibniz challenged Descartes on his views about God. See https://plato.stanford.edu/entries/ontological-arguments/