The exclusion principle is an assertion that no two electrons in an atom can be at the same time in the same quantum state. It was proposed in 1925 by Wolfgang Pauli to account for the observed patterns of light emission from atoms. Light emission is connected to the electrons' energy inside the atoms (a photon is emitted when an electron undergoes a transition to a lower energy level); and the energy of an electron is in turn connected to its quantum state. The exclusion principle has been subsequently generalised to include a whole class of particles of which the electron is only one member. These are the fermions, which include, for example, neutrons, protons and quarks.