The idea of the perfect island includes the idea of existence that is, the idea that the perfect island exists. The idea of the perfect island is the idea of a true and immutable nature. Whatever belongs to a true and immutable nature may be affirmed of it. Existence belongs to the true and immutable nature of the perfect island. The perfect island exists.
Most people find ontological arguments unpersuasive.
However, there is little agreement about where they go wrong. In 1894, he became a Hegelian because he was persuaded that a particular ontological argument is sound. Later, he came to think that the tools of modern logic could be used to show that Kant was right, and that existence is not a property. Ontological arguments were much discussed from the time of St. Anselm onwards. Many famous theologians e.g. St. Thomas Aquinas rejected these arguments.