The ontological proof of God - the attempt to prove the existence of God through pure reason, independent of all experience - serves as the keystone in the dome of rationalist metaphysics. If the keystone is removed, the vault collapses. Due to this indispensability, the ontological proof of God, first formulated by Anselm of Canterbury around the year 1100, played a decisive role in the philosophy of the 17th and 18th centuries, and to some extent in the 19th century as well. Wolfgang Röd demonstrates that the philosophical systems of Descartes, Spinoza, Leibniz, and even Hegel can only be understood by paying attention to the central position that the proof of God through pure reason occupies within them.