Researched by the Ignaria Editorial Team · Published 2026-03-12
Anselm of Canterbury argued in the Proslogion (1078) that God, defined as "that than which nothing greater can be conceived," must exist in reality as well as in the understanding — for a being that existed only in the understanding could be conceived as greater if it also existed in reality, which would contradict the original definition. This compressed argument has fascinated and divided philosophers ever since: Aquinas rejected it on the grounds that we cannot infer existence from a concept, while Descartes, Leibniz, and many analytic philosophers have defended versions of it. Gaunilo, a monk and contemporary of Anselm, offered the first counter-argument: by the same logic one could prove the existence of a perfect island.
Your research brief
This page is a summary. Your research brief is a working document.
Run the same question in Ignaria and get blockquoted primary sources you can refine, export, write from, and share.
-
Refine with Research Builder
Ask follow-up questions and deepen your line of inquiry — one brief becomes a research session.
-
Export for your projects
Pull blockquoted, cited passages into sermons, articles, and study notes.
-
Write in Ignaria
Turn your research brief into a finished article without leaving your thread.
-
Share when you're ready
Publish publicly on your profile, or share privately with colleagues on Pro.
1 free search · No account needed to start
What the primary sources show
The original statement of the ontological argument: "that than which nothing greater can be conceived" cannot exist only in the understanding, for then something greater could be conceived — namely, the same being existing in reality.
The immediate counter-argument: if Anselm's logic works for God, it should work for a "most perfect island" — whatever can be conceived as greatest must exist in reality by the same reasoning. Anselm replied that the argument applies only to a being whose non-existence is self-contradictory.