  We compute all 2-covariant tensors naturally constructed from a
semiriemannian metric which are divergence-free and have weight greater than
-2.
  As a consequence, it follows a characterization of the Einstein tensor as the
only, up to a constant factor, 2-covariant tensor naturally constructed from a
semiriemannian metric which is divergence-free and has weight 0 (i.e., is
independent of the unit of scale). Since these two conditions are also
satisfied by the energy-momentum tensor of a relativistic space-time, we
discuss in detail how these theorems lead to the field equation of General
Relativity.
