  We show that the constant time lag prescription for tidal dissipation follows
directly from the equations of motion of a tidally-forced viscous fluid body,
given some basic assumptions. They are (i) dissipation results from a viscous
force that is proportional to the velocity of the tidal flow (ii) tidal forcing
and dissipation are weak and non-resonant (iii) the equilibrium structure of
the forced body is spherically-symmetric. The lag time is an intrinsic property
of the tidally-forced body and is independent of the orbital configuration.
