Adopting a non geometrical point of view, we are led to an alternative theory of the order two and symetric gravitational tensor field of GR. The field is no more interpreted as the metric of our space-time. The true metric is globally Minkowskian and describes a flat manifold, a context which justifies a genuine rehabilitation of the global discrete space-time symetries involved in the structure of the Lorentz group along with their 'problematic' representations: the negative energy and tachyonic ones.