You could also ask him what he would expect to happen if he were to close his eyes (or turn around), and you were to smack him on the head. Sure, you hitting him would be real in your “existence”, but would he expect not to feel pain in his? That kinda goes along with floating beliefs vs. testable hypotheses; it is at least something to work with. And then, you could make it even more solipsistic by asking what happens if you both close your eyes before you smack him!
There is also the possibility of getting to understand what he thinks about other forms of sensation. You could bring up echolocation, blind people, using magnetic fields to sense, IR vision (can the soldier see you if they are looking at your heat signature through a building in pitch black? Would either of you exist in one another’s world?)… the list can be long, but it all boils down to there being evidence that you existed that whole time. If you can reach out and touch someone; with hands, or with phones, or with innernets. If they can upload images on facebook, even when they left your country, and you can see pictures of them. Then you are not being honest of your meaning of ‘existence’ when you say “they don’t exist to me”.
You could also ask him what he would expect to happen if he were to close his eyes (or turn around), and you were to smack him on the head. Sure, you hitting him would be real in your “existence”, but would he expect not to feel pain in his? That kinda goes along with floating beliefs vs. testable hypotheses; it is at least something to work with. And then, you could make it even more solipsistic by asking what happens if you both close your eyes before you smack him!
There is also the possibility of getting to understand what he thinks about other forms of sensation. You could bring up echolocation, blind people, using magnetic fields to sense, IR vision (can the soldier see you if they are looking at your heat signature through a building in pitch black? Would either of you exist in one another’s world?)… the list can be long, but it all boils down to there being evidence that you existed that whole time. If you can reach out and touch someone; with hands, or with phones, or with innernets. If they can upload images on facebook, even when they left your country, and you can see pictures of them. Then you are not being honest of your meaning of ‘existence’ when you say “they don’t exist to me”.