Well that's it isn't it. The ability to listen to others and alter your own ideas in light of the evidence.. people with delusions can't. And they don't notice or care what impact (or otherwise) they have on other people.

(And I don't want to hear anything about majorities not always being right because I don't think that's relevant? Perhaps majorities can believe things that aren't factually true. But they can't be deluded??)