“I don't know whether I was then a man dreaming I was a butterfly, or whether I am now a butterfly, dreaming I am a man.”Zhuangzi · Zhuangzi · -300 · capitolul 2
Any check of reality is made from within one state, so no one can prove which state is real.
Zhuangzi dreams he is a butterfly and, waking, finds he cannot prove which of the two states is the real one. The mechanism is that any verification is performed from within a particular state, and is therefore biased: from inside a dream, the dream is real. Someone who radically changes their life at forty and no longer recognizes the person they were at twenty lives exactly this undecidability.
Why it matters It loosens the anxiety of a fixed identity: if you cannot prove who you are with absolute certainty, you can rest more easily with change and the unknown.