See, here's the thing that gets me. You know you exist. Whether by simulation or some other method you know that you can think and feel and are to some extent real.
But no one else can ever prove to you that they are real and that they exist in the same capacity as you do. No one else can prove to you that they are not part of the dream, simulation, etc.
So I know I exist, but I don't know that you exist. And you know you exist, but you don't know if I exist. Even though I'm telling you right now that I know that I exist, I can't prove it to you, just like you can't prove it to me.