  I present a selection of conceptual and mathematical problems in the
foundations of modern physics as they derive from the title question.
Contribution to a panel session, "Springer Forum: Quantum Structures --
Physical, Mathematical and Epistemological Problems", held at the Biannual
Symposium of the International Quantum Structures Association, Liptovsky Jan,
September 1998. To appear in journal: Soft Computing (2001).
