  Several other factors, besides the intrinsic local geometry, contribute to
give a meaning to a space-time model. The simplest example comes from comparing
Minkowski's and Milne's model, that both have a null Riemann tensor. We add to
these two models a third one which describes a time-dependent
locally-Minkowskian spherically symmetric space-time on which every
test-particle at rest with respect to the center of symmetry sustains a
constant force. Although the model is globally grossly un-realistic we think
that it can be helpful to describe a local perturbation of an homogeneous
cosmological model. Or as a substitute to the very far away asymptotic
Minkowskian behavior usually assumed to describe the gravitational field of
compact spherical bodies.
