  We show that general infrared modifications of the Einstein-Hilbert action
obtained by addition of curvature invariants are not viable. These
modifications contain either ghosts or light gravity scalars. A very specific
fine-tuning might solve the problem of ghosts, but the resulting theory is
still equivalent to a scalar-tensor gravity and thus gives a corrupted picture
of gravity at the solar system scale. The only known loophole is that the
theory becomes higher dimensional at large distances. The infinite number of
degrees of freedom introduced in this way is not reducible to the addition of
an arbitrary function of curvature invariants.
