The quantum-mechanical principle, applying to fermions but not to bosons, that no two identical particles in a system, such as electrons in an atom or quarks in a hadron, can possess an identical set of quantum numbers. It was first formulated by Wolfgang Pauli in 1925. It has many important consequences in physics and chemistry, such as explaining why the periodic table occurs. The origin of the Pauli exclusion principle lies in the spin–statistics theorem of relativistic quantum field theory.