If you put a banner in front of a mirror, it flips the left-right axis, but not the down-up axis. Why don't all directions have equal merit in the universe? How does this relate to the time dimension -- is it also special in the same sense?
The reason I'm asking this is that my intuition is that every dimension should be the same. That left and right should be the same as up and down, and that forward in time should be the same as forward to the right. But nature seems to treat different dimensions differently. Here I gave the example of mirrors, that treat left and right differently from up and down. One could easily have given time: you can go forward in time, but backwards is impossible. Why does nature treat different dimensions differently? And in particular, who would a mirror, which is perfect symmetric, treat different dimensions differently? Why does nature "pick favorites" when it comes to dimensions, if it has no a priori reason to give different roles to each dimension?