Bostrom has famously argued that we live in a simulation. One of his key assumptions is that other civilizations exist that can simulate us. Why do we make this assumption?
Why assume that other civilizations exist? We obviously would not be able to see them in our universe. If we did, that would imply that we do, in fact, live in "base reality." If that would be the case (that we live in base reality, yet it is simulated by others in this base reality), it would imply an infinite chain of causality, which seems dubious at best.
Why, then, assume that other universes exist besides ours? We simply do not know if 1) other universes exist at all, and 2) life can be supported in that universe, much less intelligent life. It doesn't seem that probability can be applied here, either. How can one possibly, rationally, assign probability to this?
As a bit of an aside, it seems that the argument also assumes reductive physicalism. Is this not contested heavily?