  There is a venerable position in the philosophy of space and time that holds
that the geometry of spacetime is conventional, provided one is willing to
postulate a "universal force field". Here we ask a more focused question,
inspired by this literature: in the context of our best classical theories of
space and time, if one understands "force" in the standard way, can one
accommodate different geometries by postulating a new force field? We argue
that the answer depends on one's theory. In Newtonian gravitation the answer is
"yes"; in relativity theory, it is "no".
