There are different versions, but they all argue something like: because we can imagine a perfect being, there must be a god. The idea is that existing makes a good thing better than one that's only imaginary. So the perfect thing we're imagining must exist. Then we call the perfect thing God. The earliest objection was that an argument like that could prove wrong things. You could prove that a perfect island must exist, for example. But no real island is perfect.
Because it starts with imagination, not what you can see or experience, this is a kind of a priori reasoning. David Hume didn't like that way of thinking. He believed that knowledge had to come from experience and called everything else "nothing but sophistry and illusion".
Other versions of the argument start with the idea of the universe, and from that argue that there must be a god.
- Anselm of Canterbury in the 11th century, according to Miroslaw Szatkowski (ed) 2012. Ontological proofs today. Ontos Verlag, p22.
- "Gaunilo". Encyclopaedia Britannica.
- Hume, David 1777 . An enquiry concerning human understanding. London: A. Millar, p166.