  It has been suggested heuristically by Unruh and Wald, and independently by
Page, that among systems with given energy and volume, thermal radiation has
the largest entropy. The suggestion leads to the corresponding universal bound
on entropy of physical systems. Using a gedanken experiment we show that the
bound follows from the second law of thermodynamics if the CPT symmetry is
assumed and a certain general condition on matter holds. The experiment
suggests that a wide class of Lorentz invariant local quantum field theories
obeys a bound on the density of states.
