I think I have an idea which might drive you mad
My guess is that this is probably a question that the majority of INTPs have been pondering over the course of their lives already. I was a young teenager when I came up with it, and I thought it was really profound back then myself. Until I learned that it was kinda old news from a philosophical standpoint. That was rather a humbling moment . . .
Anyway, here's my take on it for somewhere around 25 years . . . it's way more complicated than this, but I thought I'd stick to the basics so I don't bore you to death.
Everything I experience - every physical sensation, every thought, every tiny piece of evidence about anything in the world - takes place in my mind. Because of that, I can't even be certain that *I* exist, at least not in any kind of physical way.
So the world I experience is 100% about me. Before you think I'm just being egotistical, consider that this would be true for anyone else who happened to exist in the same way I do.
If something happens that I am unaware of,
it does not exist in my world. It only comes into existence in my world when I find out about it. The same is true for people. People I don't know, don't hear about, and don't run into somewhere do not exist in my world.
When I die, this world that I live in ends. (Whether I go onto some other kind of existence is something I don't know, of course.)
Note that when contemplating this from the perspective of my own personal experience, there isn't a question about whether or not other people exist. If I perceive them to, they exist. If I don't, they don't. Period.
That does still leave the question of whether there are other conscious beings who have their own "worlds" just like I do. And if there is some physical realm that they share with me. Maybe the same one I perceive through my senses? That's what most people accept, but I can't be too sure.
It can kind of torture you if you start thinking about it too much, but on the other hand I think it's kind of fun not to know for sure and to think about all the interesting possibilities.