Imagine you are a duck. You swim. You fly. You eat stuff ducks eat. You mate. You sleep. If you're female, you give birth to eggs which become babies that you raise until they can go off and be their own ducks. You defend your territory, and protect yourself from enemies. You occasionally migrate to a place your instincts say exists.

Anything your instincts don't believe in, and that you cannot sense, does not exist, or matter. Because you live in the reality lived in by ducks.

Not to say all reality is subjective, but that a certain amount of our reality is formed from experience with life based off what life gives us.