In quantum mechanics, indistinguishable particles are particles that cannot be distinguished from one another, even in principle. Species of identical particles include, but are not limited to, elementary particles, composite subatomic particles, as well as atoms and molecules. Although all known indistinguishable particles only exist at the quantum scale, there is no exhaustive list of all possible sorts of particles nor a clear-cut limit of applicability, as explored in quantum statistics. They were first discussed by Werner Heisenberg and Paul Dirac in 1926.