The traditional goal of particle physics has been to identify what appear to be structureless units of matter and to understand the nature of the forces acting between them; all other entities are then to be successively constructed as composites of these elementary building blocks. The electron was recognized both as the ‘particle of electricity’ – or as we might now say, as an elementary source of the electromagnetic field, with its motion constituting an electromagnetic current – and also as an important constituent of matter. The electromagnetic interactions of the mu and the tau leptons are the same as for the electron. In weak interactions, each charged lepton is accompanied by its ‘own’ neutral partner, a neutrino. The conclusion would now seem irresistible that such spectra should be interpreted as the energy levels of the systems of bound constituents.