"In classical mechanics... you can always tell the particles apart, in principle - just paint one of them red and the other one blue, or stamp identification numbers on them, or hire private deterctives to follow them around. But in quantum mechanics the situation is fundamentally different: you can't paint an electron red, or pin a label on it, and a detective's observations will inevitable and unpredictable alter the 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 is not merely that we don't know which electron is which; God doesn't know which is which, because there is no such thing as "this" electron or "that" electron..."

David Griffiths, Introduction to Quantum Mechanics

Coherence Bosons and fermions