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How did Anselm of Canterbury contribute to arguments about God's existence?

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Painting by Joseph Aubert of Saint Thomas Aquinas seated with a book, wearing a halo and black and white robes, surrounded by ornate patterns in St. Stephens Church, Jerusalem. — Western philosophy Painting of scholastic philosopher St. Thomas of Aquinas by Joseph Aubert, 1900.

In the history of Western philosophy, St. Anselm of Canterbury is best known for formulating the ontological argument for the existence of God.12

The Ontological Argument

Anselm’s argument, presented in his Proslogion, proceeds a priori—from the idea of God rather than from observation of the world. He defined God as "that than which nothing greater can be conceived". Anselm argued that if this being exists only in the mind, it would not be the greatest possible being, because a being that exists in both the mind and reality is greater than one that exists in the mind alone. Therefore, to remain consistent with the definition of unsurpassable perfection, God must exist in reality.1342

Other Contributions

Before the Proslogion, Anselm authored the Monologion, which provided three proofs for God’s existence based on Neoplatonic thought. These arguments move from the observation of multiple good or perfect things to the existence of a single, supreme source of goodness and perfection.2

Anselm’s work remains a central subject of philosophical debate, having faced early opposition from Gaunilo and later critiques regarding the nature of existence as a predicate.54