  I compute from first principles the local heating rate $q$ (the amount of
electromagnetic energy converted to heat per unit time per unit volume) for
electromagnetic waves propagating in magnetically and electrically polarizable
media. I find that, in magnetic media, this rate has two separate
contributions, $q^{(V)}$ and $q^{(S)}$, the first coming from the volume of the
medium and the second from its surface. I argue that the second law of
thermodynamics requires that the volume contribution be positive and that this
requirement, in turn, prohibits negative refraction. This result holds for
active or passive media and in the presence of anisotropy and spatial
dispersion.
