  We show that generalizations of general relativity theory, which consist in
replacing the Hilbert Lagrangian $L_{Hilbert} = \frac 1{16\pi} \sqrt{|g|} R$ by
a generic scalar density $L=L(g_{\mu\nu}, R^\lambda_{\mu\nu\kappa})$ depending
upon the metric $g_{\mu\nu}$ and the curvature tensor
$R^\lambda_{\mu\nu\kappa}$, are equivalent to the conventional Einstein theory
for a (possibly) different metric tensor $\tilde{g}_{\mu\nu}$ and (possibly) a
different set of matter fields. The simple proof of this theorem relies on a
new approach to variational problems containing metric and connection.
