  We analyse the "Einstein box" thought experiment and the definition of the
momentum of light inside matter. We stress the importance of the total
energy-momentum tensor of the closed system (electromagnetic field plus
material medium) and derive in detail the relativistic expressions for the
Abraham and Minkowski momenta, together with the corresponding balance
equations for an isotropic and homogeneous medium. We identify some assumptions
hidden in the Einstein box argument, which make it weaker than it is usually
recognized. In particular, we show that the Abraham momentum is not uniquely
selected as the momentum of light in this case.
