There is a philosophical theory about whether things truly exist or not, called solipsism. The main thrust of this theory that we can only be sure of our own existence, not of the existence of anything or anyone outside of us.
A truly solipsistic person can never know whether the people that they meet are real or just figments of their imagination. For all the solipsist knows, they are just a brain floating in a tank, experiencing whatever stimuli are zapped into their vestigial spinal cord.
Solipsists are rarely fun at parties, not least because they can’t be sure that anyone else has actually shown up.
Perhaps that’s a little unfair to solipsism, but this theory brings up an interesting question.
Can we ever be sure that anyone else experiences the world the way we do?