  We propose a simple theory for the ``universal'' scaling law previously
reported for the distributions of waiting times between earthquakes. It is
based on a largely used benchmark model of seismicity, which just assumes no
difference in the physics of foreshocks, mainshocks and aftershocks. Our
theoretical calculations provide good fits to the data and show that
universality is only approximate. We conclude that the distributions of
inter-event times do not reveal more information than what is already known
from the Gutenberg-Richter and the Omori power laws. Our results reinforces the
view that triggering of earthquakes by other earthquakes is a key physical
mechanism to understand seismicity.
