And they never notice that the hallway is infinatly tall.

But yeah, nice touch with having depth be the fourth dimension instead of time.

so... if <- and ^ are the first and second dimension... and depth is the fourth... what's the third? Or rather... what are the "three" demensions in this case?

I sorta assumed they saw themselves in three dimensions, and the 'fourth' was the fact that this dude could see outside the frame of the comic. A bit like the so called fourth wall.

Also, for anybody interested, I consider there to be infinite spatial dimensions, one being time, I guess I'll concede. We obviously only expereience three dimensions of motion and time, but that doesn't mean theres not a huge ammount of other spatial dimensions around use we can't even perceive.! Little interesting notion about how I consider the universe there I guess.