animal(seagull)        has_feathers(seagull)                                   ∀ X. ( animal(X) ∧ has_feathers(X) → bird(X) )
  -------------------------------------------------------- (∧-I)         ---------------------------------------------------------- (∀-E)
                animal(seagull) ∧ has_feather(seagull)                    animal(seagull) ∧ has_feathers(seagull) → bird(seagull)
              ------------------------------------------------------------------------------------------------------------------------ (MP)
                                                            bird(seagull)
                                                      ---------------------- (∃-I)
                                                            ∃X. bird(X)