  It is shown that the Dirac sea can be uniquely defined for the Dirac equation
with general interaction, if we impose a causality condition on the Dirac sea.
We derive an explicit formula for the Dirac sea in terms of a power series in
the bosonic potentials.
  The construction is extended to systems of Dirac seas. If the system contains
chiral fermions, the causality condition yields a restriction for the bosonic
potentials.
