  We discuss the roles of viscosity in relativistic fluid dynamics from the
point of view of memory effects. Depending on the type of quantity to which the
memory effect is applied, different terms appear in higher order corrections.
We show that when the memory effect applies on the extensive quantities, the
hydrodynamic equations of motion become non-singular. We further discuss the
question of memory effect in the derivation of transport coefficients from a
microscopic theory. We generalize the application of the Green-Kubo-Nakano
(GKN) to calculate transport coefficients in the framework of projection
operator formalism, and derive the general formula when the fluid is
non-Newtonian.
