It's generally useful to behave as if we all live in the same universe. Even if one agrees with quantum solipsism - the idea that one is the only person one knows exists and that the existence of the entire universe depends on you continuing to exist - walking around acting that way makes you look crazy and you're sure to piss off everyone around you if they do in fact also exist. Life is a lot easier to live if one accepts the postulate that other people really do exist even if it can't be proven - and even if they don't exist, behaving as if they do keeps your experience of reality consistent with the expectations your mind has built up over years of experience before you started questioning the reality of other people. If one puts aside quantum solipsism, there is still a problem in knowing whether or not other people exist in the same universe as you or not. It turns out that one can only prove that other people's universe is consistent with your universe, an
I go through life looking at everything objectively and rationally, whether I like it or not. You can come too.