 People are especially interesting, but metaphysical questions about persistence have been raised for objects of various other kinds, including musical works, social entities such as institutions or groups of people, and biological species. It is often controversial what sort of entities these are – material objects, abstract types, processes, sets? – and so it is also controversial how far the arguments discussed above, which focus on material objects, are relevant to these special cases. Useful articles elsewhere in this Encyclopedia include  Philosophy of Music,  Philosophy of Theater,  Biological Individuals,  and  Social Ontology.