  A new integral identity for functions with continuous second partial
derivatives is derived. It is shown that the value of any function f(r,t) at
position r and time t is completely determined by its previous values at all
other locations r' and retarded times t'<t, provided that the function vanishes
at infinity and has continuous second partial derivatives. Functions of this
kind occur in many areas of physics and it seems somewhat surprising that they
are constrained in this way.
