Ontological argument
The ontological argument is an idea in religious philosophy. It is supposed to show that God exists.
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.[1] 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.[2]
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".[3]
Other versions of the argument start with the idea of the universe, and from that argue that there must be a god.