What's a mirror? At night, when you wash your teeth in front of the mirror, you can see your face. Then you go to sleep. The morning after you wash you teeth again, so you see your face again and it's different from the last face you saw, maybe because of the pillow. Anyway you can recognise yourself because the difference is very slight. And if this difference was not so slight? Say, you wake up and see a completly different face (with different eyes and nose) in the mirror. Can you recognise yourself? Yes, of course. There's something inside us that allows us to know we are there (in front of the mirror) for any image you can see in that mirror... It's you and you know it. Therefore, there's a staff inside us (maybe in the brain) which doesn't change over time unlike our body. PS: Today I've put on the jacket of the last year. This way I'm sure I can recognise me.