There are people who believe we live in a world, simulated on a computer. That computer must have been built in either another computer-generated world or a real world (by which I mean a non-simulated world). If it has been made in another computer-simulated world then the (simulated) computer on which the simulation (program) for "the next" simulated world is run must be a subset (by which I circumvent the notion of size) of the "previous" computer, which means that the computers on which the next-world simulations are run will be ever-decreasing subsets of the previous computer.
Doesn't this mean that there has to be a first real world because if the "stack of turtles" would be infinite there would be no limit to the increasing subset connected to a computer in a previous simulated Universe, sending the subset to infinity, when the simulated Universe gets more and more down the stack of turtles?
If the simulated Universe grows to an infinite subset (when tracing back the track of subsequent simulations or going down the stack of turtles) of the previous computer, whatever the programmers program into it, wouldn't it take an infinite time to program this infinite set, so the next simulated Universe never gets finished (and neither the preceding simulation) and the process stops. In this case, there has to be a first, infinite real-world (because such a world can't be programmed in an infinite computer). Programmers in this real infinite world can make a finite-world simulation.
This is a crude (but essential) outline of my thoughts on this subject, of which I clearly think that a real world has to be there.
By the way, I think it's a waste of time to elaborate on the idea that we live in a simulated world (which indeed, as written below only shifts the problem of explaining Nature to an alleged previous simulated world, etc.). I'm satisfied with the world I live in and it's my gut feeling that the world I live in is real. So in my eyes, there isn't a first world that's real but it's the world we live in that's real (I think it's a creepy thought that we live in a simulated world, just I think it's a creepy thought that we're made by (a) god(s) or by evolution for that matter). But despite all that, I'm still curious how others on this site think about this subject.