Solipsism is a philosophical belief that only one's mind is certain to exist. For exemple: you who are reading this, you think therefore you are, you have a conscious so you exist. But if you can prove your own existence, can you prove others'?
After doing some research, I found that some villains hold this belief (and maybe more, tell me if they're not among the one that I'm going to list):
There's probably more, so tell me, and maybe it'll be necessary to create this category.