Interaction defines existence.
I’m not sure what the old philosophy is, other than to say, if something exists only for you, then you’re just crazy or foolish. If it exists for everyone, but for no thing, it’s mass hysteria. But if things in the world that we interact with react to it, then we call it real.
But maybe we’re just imagining that, too.
“Religion”: a belief system shared by a lot of people
“Cult”: a belief system shared by only a few people
“Insanity”: a belief system unique to one individual.
“A lot” and “A Few” of course being a matter of politics and armed might as much as reason or logic.