  We explore the consequences of assuming that the bounded space-time subsets
contain a finite number of degrees of freedom. A physically natural hypothesis
is that this number is additive for spatially separated subsets. We show that
this assumption conflicts with the Lorentz symmetry of Minkowski space since it
implies that a conserved current determines the number of degrees of freedom.
However, the entanglement across boundaries can lead to a subadditive property
for the degrees of freedom of spatially separated sets. We show that this
condition and the Poincare symmetry lead to the Bousso covariant entropy bound
for Minkowski space.
