 The starting point for the idea of continuous internal symmetries was the interpretation of the presence of particles with (approximately) the same value of mass as the components (states) of a single physical system, connected to each other by the transformations of an underlying symmetry group. This idea emerged by analogy with what happened in the case of permutation symmetry, and was in fact due to Heisenberg (the discoverer of permutation symmetry), who in a 1932 paper introduced the SU(2) symmetry connecting the proton and the neutron (interpreted as the two states of a single system). This symmetry was further studied by Wigner, who in 1937 introduced the term isotopic spin (later contracted to isospin). The various internal symmetries are invariances under phase transformations of the quantum states and are described in terms of the unitary groups SU(N). The term “gauge” is sometimes used for all continuous internal symmetries, and is sometimes reserved for the local versions (these being at the core of the Standard Model for elementary  particles).[14]