I do agree with emeraldfool in this discussion. But it's actually a definition problem. If you believe in one absolute "true world" in which we all live and of which we all see different parts, Ithorius and Bilbous are completely right. If I think the world is flat, it still isn't flat in the "true world".
If you, on the other hand, drop the idea of a "true world" and say that world is only what we see and hear about it, then the people really lived on a flat word, 1000 year ago. If you take it that way, everybody lives in a different world. If someone thinks he is Napoleon, then for him he really is. For all the other people, he's not, but truth is no democracy.
I think it's fair to drop the "true world" though. Someone who is blind lives in a different world, even if he lives on the same planet. Our senses are limited, so in a way we are all blind. Our world as we know it, is only what our brain in combination with our senses can derive from it. It will always be a derivative. That's why the concept of a "the true" world is rather useless, as we can never quantify it. I would be 100% empty.
If you say "the people back then lived on a round world, even if they thought they lived on a flat world, because now we know that the world is round, because we saw it with our satellites" you're pasting personal knowledge on the "true world" and extrapolate it. The question is if that's valid. We can never constitute a perfect "true world" or even parts of it, when all we have is dirty info. Especially since we don't know in which way the info is blurred and shifted. I'd say it's fair to neglect the "true world" since we don't even know if it really exists.
This is not only a wacky philosophy; modern science doesn't use a "true world" theory either. It turns the idea of a true world on its head. In science, something is true as long as it describes the world as we see it in the best way. If a new theory describes the world better, we would consider the old theory to be false and the new one to be true. Scientific truth changes and starts with personal experience. Scientific truth accepts the fact that it's not perfect but that it's 'the best truth we have'. It is valid to consider science a way of approaching the "true world" though.
End of rant, I hope it made some sense
