The Argument for the Existence of Hawaii

Blom, Philipp. Nature's Mutiny: How the Little Ice Age of the Long Seventeenth Century Transformed the West and Shaped the Present (p. 153). Liveright. Kindle Edition. 

Descartes (via Anselm) argued that: 
A Perfect Being must really exist, because if it had no existence, it would be imperfect. Therefore, a Perfect Being exists, unlimited in time and space (otherwise, its perfection would be destroyed), and hence omnipresent and eternal, the origin of all being, the creator of this temporal, material world. Moreover, the idea of perfection also means that a Perfect Being must be be truthful, because any lie would be falsehood, incoherence, imperfection, and therefore irreconcilable with its perfect nature.

Blom, Philipp. Nature's Mutiny: How the Little Ice Age of the Long Seventeenth Century Transformed the West and Shaped the Present (p. 152-153). 
Liveright. Kindle Edition. 
the monk Gaunilo of Marmoutiers, who cheekily asked whether, if he could conceive of a perfect island in the ocean, this island would also necessarily have to exist.

Blom, Philipp. Nature's Mutiny: How the Little Ice Age of the Long Seventeenth Century Transformed the West and Shaped the Present (p. 153). Liveright. Kindle Edition.