  The heuristic principle that constructive dynamics may arise wherever there
exists a difference, or gradient, is discussed. Consideration of black-hole
entropy appears to provide a clue for setting a lower bound on any extensive
measure of such collective system difference, or potential to give rise to
constructive dynamics. It is seen that the second-power dependence of
black-hole entropy on mass is consistent with the difference principle, while
consideration of Hawking radiation forces one to beware of implicit
figure-ground distinctions in the application of the difference principle.
