[bheadmaster]

Depends on how you define "existence".

In general, we define existence in context of consistence of stimulus with everyday experience, including other people's reports of experience. Dreams are very rarely consistent with each other, so we know they're dreams. But imagine if every time you went to sleep, you woke up in a "dreamworld" with all the experience sequentially consistent with your previous dreams. How would you know that the "dream" is the dream, and that the real world is the real world?

I'm not saying that hallucinations are real - I strongly believe the opposite. But I can see such how powerful and consistent hallucinations could make the person believe in their existence, especially if combined with low life satisfaction/self-esteem and strong desire to feel special in some sort of way, to know something that the "unenlightened" don't, or to be a part of something. It's a similar mechanism to one that makes people join religions - psychedelic experiences have a lot in common with religious experiences. One could also hypothesize that religion was invented when humans found psychedelics, and couldn't explain their effects.