Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             chemical compounds
               Class phosphines [R3P]
                 1 "phosphine"ᵀ [PH3]
                 2 primary phosphines [RPH2 (R≠H)]
                 3 secondary phosphines [R2PH (R≠H)]
                 4 tertiary phosphines [R3P (R≠H)]