A recent paper by Hartle [Phys. Rev. D 51, 1800 (1995)] proposes a definition
of "spacetime information" - the information available about a quantum system's
boundary conditions in the various sets of decohering histories it may display
- and investigates its properties. We note here that Hartle's analysis contains
errors which invalidate several of the conclusions. In particular, the proof
that the proposed definition agrees with the standard definition for ordinary
quantum mechanics is invalid, the evaluations of the spacetime information for
time-neutral generalized quantum theories and for generalized quantum theories
with non-unitary evolution are incorrect, and the argument that spacetime
information is conserved on spacelike surfaces in these last theories is
erroneous. We show however that the proposed definition does, in fact, agree
with the standard definition for ordinary quantum mechanics. Hartle's
definition relies on choosing, case by case, a class of fine-grained consistent
sets of histories. We supply a general definition of the relevant class of sets
that agrees with Hartle's definition in the cases explicitly considered and
that generalizes to other cases.Comment: 11 pages, LaTeX with ReVTeX. References update
