  I present an analysis of the physical assumptions needed to obtain the metric
structure of space-time. For this purpose I combine the axiomatic approach
pioneered by Robb with ideas drawn from works on Weyl's "Raumproblem". The
concept of a Lorentzian manifold is replaced by the weaker concept of an "event
manifold", defined in terms of volume element, causal structure and affine
connection(s). Exploiting properties of its structure group, I show that
distinguishing Lorentzian manifolds from other classes of event manifolds
requires the key idea of general relativity: namely that the manifold's
physical structure, rather than being fixed, is itself a variable.
