Suppose one day we are able to fully simulate a human brain such that it believes it is living in a fully coherent real universe with other real people in it but in reality that simulated brain is the only real thing. That brain looked around at everything and came to the conclusion that it was living in the real world with real other people. That’s the conclusion the brain should come to given the evidence but it in fact was very wrong.

My question is who’s to say I’m not currently that brain? Why is it treated as if that’s a ridiculous question? If the experiences are by definition indistinguishable why is it anything better than a 50/50 shot that I’m living in the real world with real other people?