Solipsism and Idealism.
Solipsism is the fact that you can never directly know anything about anyone else’s inner world, about their experience. There is absolutely no way to prove that other people have an inner world. That they experience anything. The lights could be off and you would never know. Perhaps this, your experience, is the only experience. How can you know for sure, without assuming, believing, guessing, inferring that other people aren’t NPC’s (non-playable characters)?
Idealism is the fact that you cannot escape consciousness. Everything and anything you can experience, you can only experience in consciousness. Whatever you see, feel, hear, smell or think, those are all experiences in consciousness. Pick up a rock and feel its hardness, see its shape? You just had a bunch of experiences in consciousness. How can you know, for sure, that there actually was a rock beyond those experiences? How can you know that there is matter, that there is a universe, that anything exists beyond experience, beyond consciousness?
You cannot.
Solipsism and idealism are epistemic boundaries. They are limits to knowledge. Going beyond them requires belief. Belief that there are more inner worlds than the one you experience. Belief that there is a universe out there, beyond experience in consciousness.
Belief!
Solipsism and idealisms aren’t beliefs. They are limits to knowledge. They are what can be known directly and everything else is belief. And yes, that includes other people, matter and the universe.
Like the richest man on earth once said: ‘Let that sink in.’