  Our understanding of the four basic concepts of Physics -- space, time,
matter and force -- has undergone radical change in the course of work on
unification, starting with Maxwell's unification of electricity with magnetism,
all the way to present day string theory. What started as four independent
concepts, with space and time postulated and the possible forms of matter and
force arbitrarily chosen, now appear as different aspects of a rich and novel
dynamically determined structure.
