Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       particles
         elementary particles
           elementary fermions - elementary particles with half-integer spin
             leptons
               1.1 electron 1897
               1.2 electron neutrino C.L. Cowan et al., 1956
               2.1 muon 1930's
               2.2 muon neutrino 1962
               3.1 tau M.L. Perl et al., 1975
               3.2 tau neutrino