A fundamental theorem of relativistic quantum field theory that states that half-integer spins can only be quantized consistently if they obey Fermi–Dirac statistics and even-integer spins can only be quantized consistently if they obey Bose–Einstein statistics (see quantum statistics). This theorem enables one to understand the result of quantum statistics that wave functions for bosons are symmetric and wave functions for fermions are antisymmetric. It also provides the foundation for the Pauli exclusion principle. It was first proved by Pauli in 1940 and has been proved in several different ways since then.