  The perturbative construction of the S-matrix in the causal spacetime
approach of Epstein and Glaser may be interpreted as a method of regularization
for divergent Feynman diagrams. The results of any method of regularization
must be equivalent to those obtained from the Epstein-Glaser (EG) construction,
within the freedom left by the latter. In particular, the conceptually
well-defined approach of Bogoliubov, Parasuk, Hepp, and Zimmermann (BPHZ),
though conceptually different from EG, meets this requirement. Based on this
equivalence we propose a modified BPHZ procedure which provides a significant
simplification of the techniques of perturbation theory, and which applies
equally well to standard quantum field theory and to chiral theories. We
illustrate the proposed method by a number of examples of various orders in
perturbation theory. At the level of multi-loop diagrams we find that
subdiagrams as classified by Zimmermann's forest formula in BPHZ may be
restricted to subdiagrams in the sense of Epstein-Glaser, thus entailing an
important reduction of actual computations. Furthermore, the relationship of
our approach to the method of dimensional regularization is particularly
transparent, without having to invoke analytic continuation to unphysical
spacetime dimension, and sheds new light on certain parameters within
dimensional regularization.
