I could copy paste some lines from Terry Pratchett's Wytches Abroad right here... Nvm, just have the synopses : warning, spoilers if you haven't read any Pratchett so far - on the other hand, there's a whole series of these books that just transpires awesomeness.
In the mirror universe, Granny and Lilith are both confronted by endless reflections. Death tells them that they are both alive and dead, and can only escape when they find the one version of themselves that is real. Granny simply looks down at herself and says,
"This one."
Lilith, whose whole life has consisted of reflections, is unable to choose and is doomed to spend the rest of time trapped.
It's a metaphor, for the masks - just to develop the point, Lilith Weatherwax practices some sort of mirror magic, feeding her soul into mirrors, therefore she loses herself, whilst Granny Weatherwax, or Esme, (Esmeralda) just knows she's her self - which would appear plain obvious to her, being a headology user. Some terms here might seem obscure, but you know... I can clarify some stuff if you want. = ) If ever.