With this volume a new series of publications edited by the Accademia Nazionale dei Lincei and the Scuola Normale Superiore begins, which collect the lectures held by some guest professors of Lincei (Ospiti Lincei) at the Scuola Normale. The Lezioni Fermiane series' aim is to provide some books of a high scientific level, which are the original synthesis of particularly interesting arguments, to the international scientific community.