We present here arguments in favor of the existence of the lightest lepton and its neutrino. This new family with a fourth lepton flavor uncovered so far unobserved universal properties of matter. The unity of the laws of their nature admits the flavor symmetrical decays of the electron and proton. Thereby, it predicts the new modes in the decays of the muon, tau lepton and neutron. At the same time, in all these transitions, no conservation laws are violated, confirming the availability in nature of a place for an additivity of mass.