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We review the traditional derivation of the fluid-dynamical equations fromkinetic theory according to Israel and Stewart. We show that their procedure toclose the fluid-dynamical equations of motion is not unique. Their approachcontains two approximations, the first being the so-called 14-momentapproximation to truncate the single-particle distribution function. The secondconsists in the choice of equations of motion for the dissipative currents.Israel and Stewart used the second moment of the Boltzmann equation, but thisis not the only possible choice. In fact, there are infinitely many moments ofthe Boltzmann equation which can serve as equations of motion for thedissipative currents. All resulting equations of motion have the same form, butthe transport coefficients are different in each case.

Derivation of fluid dynamics from kinetic theory with the 14--moment approximation