  Assuming that the Hamiltonian of a canonical field theory can be written in
the form N H + N^i H_i, and using as the only input the actual choice of the
canonical variables, we derive: (i) The algebra satisfied by H and H_i, (ii)
any constraints, and (iii) the most general canonical representation for H and
H_i. This completes previous work by Hojman, Kuchar and Teitelboim who had to
impose a set of additional postulates, among which were the form of the
canonical algebra and the requirement of path-independence of the dynamical
evolution. A prominent feature of the present approach is the replacement of
the equal-time Poisson bracket with one evaluated at general times. The
resulting formalism is therefore an example of a classical history theory -- an
interesting fact, especially in view of recent work by Isham et al.
