Right, a good question. I did not mean a simulation of something, rather "let's set up some rules and see what happens". In the same sense that the Conway's Game of Life is a simulation of life. And yes, to those inside it is reality. And to those outside it is a small subset of reality. And my original point was that those inside can potentially infer that there must be something outside by hitting the limit of what they can explain. But they have no hope of breaking out. This last argument, of course, relies on the tenuous assumption of the physical laws being mathematical structures and every event in the world being, in essence, a theorem.