I think you're imagining a projection of a 2D world into three dimensions and correctly observing that it's three-dimensional. Imagine a 2D world with two squa

I think you're imagining a projection of a 2D world into three dimensions and correctly observing that it's three-dimensional. Imagine a 2D world with two squares and a circle. One of the squares pushes the circle and it rolls into the other square, impacting it. This is all plausible in a purely 2D setting, right? Assuming that these objects can't take up the same 2D space, it makes sense that the square would be impacted by the circle. This is how vision works — photons bounce around and our eyes sense them. There wouldn't actually be any Z-axis to what you're seeing, you'd just be registering 2D photon-equivalents moving in two dimensions. But in order to represent it as something our brains can recognize as "seeing," we have to project the input into some non-zero dimension. A different representation could emphasize this point better. You could have this same basic game, but instead of first-person, have it be third-person but with actual vision simulation, so that you only see the parts of objects that Mario would see and everything else is simply absent.