Generalized second law and the Bekenstein entropy bound in Gedankenexperiments with black holes

Gedankenexperiment with the entropy and energy exchange between a thick box and a black hole is considered. It is shown that validity of the generalized second law follows from the ordinary second one. In particular, it manifests itself in positivity of the heat capacity of a box. Our treatment does not use either the Unruh-Wald or Bekenstein entropy bound. It is demonstrated that the latter itself proves to be the consequence of laws of themodynamics.