"You can't paint an electron red, or pin a label on it, and a detective's observations will inevitably and unpredictably alter its state, raising doubts as to whether the two had perhaps switched places. The fact is, all electrons are utterly identical, in a way that no two classical objects can ever be. It's not just that we don't happen to know which electron is which; God doesn't know which is which, because there is no such thing as "this" electron, or "that" electron; all we can legitimately speak about is "an" electron."


Griffiths, David (2004). Introduction to Quantum Mechanics (2nd ed.). Prentice Hall. ISBN 0-13-111892-7.