The Mirror did not touch the ground; the golden frame had no feet. It didn't look like it was hovering; it looked like it was fixed in place, more solid and more motionless than the walls themselves, like it was nailed to the reference frame of the Earth's motion.

The Mirror is in the fourth wall. Now that we-the-readers have seen the mirror, we have to consider that our seeing Eliezer saying this isn't in the mirror might just be part of our coherent extrapolated volition.