I read somewhere that there's no such thing as a completely made up person in a dream. Like, if you see a face in a dream, it's a face you've seen somewhere in real life.

I saw that on Tumblr once, and I'm convinced it can't be right - not just because I'm sure my dreams have produced faces I've never seen before, but because I don't think there's any possible way that could reliably have been researched! How could anyone possibly prove that nobody has ever made up a face in their dreams, given that it is literally impossible for anyone to a) view someone else's dream or b) establish every face a person has ever seen?