About evolution equations.

  The heat equation is an evolution equation, in which one independent variable, ``t'', can be physically identified as time. This is of great interest, because it tells us that the temperature T at any given time can depend on everything that happened at earlier times, but not on what happens at later times. Changing, say, the boundary condition at some time t=2 does not change the temperature profile at an earlier time such as t=1.