  Liu procedure is applied to a first order weakly nonlocal special
relativistic fluid. It is shown, that a reasonable relativistic theory is and
extended one, where the basic state space contains the momentum density. This
property follows from the structure of the energy-momentum balance and the
Second Law of thermodynamics. Moreover, the entropy depends on the energy
density and the momentum density on a given specific way, indicating that the
local rest frame energy density cannot be interpreted as the internal energy,
the local rest frame momentum density should be considered, too. The
corresponding constitutive relations for the stress and the energy flux are
derived.
