  A simplified derivation of Yurtsever's result, which states that the entropy
of a truncated bosonic Fock space is given by a holographic bound when the
energy of the Fock states is constrained gravitationally, is given for
asymptotically flat spacetimes with arbitrary dimension d greater or equal to
four. For this purpose, a scalar field confined to a spherical volume in
d-dimensional spacetime is considered. Imposing an upper bound on the total
energy of the corresponding Fock states which ensures that the system is in a
stable configuration against gravitational collapse and imposing a cutoff on
the maximum energy of the field modes of the order of the Planck energy leads
to an entropy bound of holographic type. A simple derivation of the entropy
bound is also given for the fermionic case.
