Ok, so i dont believe in spirits or anything else like that, is it tru that....?

Ok, so people say they see spirits. I dont believe them. i jsut think that whoever sees these spirits are paranoid, like they think so much about them that they see them, sort of like the placebo effect. then i heard that if you do not believe in them, you will never see them. so, is my theory right? like is just in people's imaginations, or are they real and im jsut not a believer? what do u think?