It's not a matter of 'belief'.
You have either experienced it or not.
As far as life in other 'realms' yes. But it depends how you classify the term 'life'. If it's something you can't reach out and touch then it's not going to be like we humans are exactly.
But as far as things like hallucinations, visions, dreams then yes we humans can experience 'spirits' or 'entities' or what some might call aliens when enter this type of mindstate.
I see what you're saying, but belief is still part of it... Even "seeing is believing" kinda proves that really. We use belief to make sense of what we experience. Two people can experience something similar and explain it in totally different ways, depending on what they believe. One person may call something a spirit or entity, while someone else may call it a dream, hallucination, or brain chemistry. The experience can still be real without everyone agreeing on what it really means.
When I asked about life beyond our planet, I wasn't really talking about visions or altered states... I was asking whether people believe life exists somewhere else outside this planet. So yes, experience matters, but belief still plays a major role in how we label and understand it.