How did Anselm’s ontological argument attempt to prove God’s existence?

St. Anselm of Canterbury, a foundational figure in Scholasticism, formulated the ontological argument in his work Proslogion (1077–78) to prove the existence of God through reason alone.123

The argument proceeds from the definition of God as "that than which nothing greater can be conceived" (aliquid quo nihil majus cogitari possit). Anselm’s reasoning follows these steps:14

• Conceptualization: Even those who deny God's existence (the "fool") understand the concept of a being than which nothing greater can be conceived. Therefore, this being exists at least in the human intellect.3
• Comparison of Existence: It is greater to exist both in the intellect and in reality than to exist in the intellect alone.3
• The Contradiction: If the greatest conceivable being existed only in the mind, it would not truly be the greatest, as a being that also exists in reality would be greater. Therefore, to avoid a logical contradiction, this unsurpassably perfect being must necessarily exist in reality.143

This argument remains one of the most debated topics in the history of philosophy, with critics like Immanuel Kant later arguing that existence is not a predicate that can be added to a definition to prove reality.42