There's a very simple objection to the simulation hypothesis that I came up with, and I've read others come up with this same idea. So I assume it's something philosophers should already be aware of.
There are 2 possibilities:
We're in a simulation. In which case we cannot infer anything about the world or worlds above our own (be it also a simulated world or non-simulated real world). We can only make inferences about simulations that will be constructed within our own world.
We're not a in a simulation. In which case there is no higher level world to make inferences about. We can only make inferences about simulations we may construct within our world.
Either way we can only make inferences about simulations constructed 'within' our world. We can't make any inferences about whether there's a world above our own. So all of Bostrom's arguments tell us nothing about the probability that our world is itself a simulation or not. All of Bostrom's references to worlds/civilizations have to refer to world/civilizations within our own. So when he calculates probabilities he's restricted to worlds under our own.
So Bostrom's paper is a bunch of elaborate reasoning about how worlds constructed within our world are probably simulations. It's really a bunch of useless reasoning, because worlds under our own are by definition simulations.
What is the response from philosophers to this objection?
Here's a link to his paper: