  We argue that, in contrast to the non-relativistic approach, a relativistic
evaluation of the nucleon--hole and delta-isobar--nucleon hole contributions to
the pion self-energy incorporates the s-wave scattering, which requires a more
accurate evaluation. Therefore relativistic approach containing only these
diagrams does not describe appropriately the pion self-energy in isospin
symmetric nuclear matter. We conclude that, a correct relativistic approach to
the pion self-energy should involve a more sophisticated calculation in order
to satisfy the known experimental results on the near-threshold behaviour of
the pion-nucleon (forward) scattering amplitude.
