  A discussion of the meaning of a physical concept cannot be separated from
discussion of the conditions for its ideal measurement. We assert that
quantization is no more than the invocation of the quantum of action in the
explanation of some process or phenomenon, and does not imply an assertion of
the fundamental nature of such a process. This leads to an ecumenical approach
to the problem of quantization of the gravitational field. There can be many
valid approaches, each of which should be judged by the domain of its
applicability to various phenomena. If two approaches have overlapping domains,
the relation between them then itself becomes a subject of study. We advocate
an approach to general relativity based on the unimodular group, which
emphasizes the physical significance and measurability of the conformal and
projective structures. A discussion of the method of matched asymptotic
expansions, and of the weakness of terrestrial sources compared with
astrophysical and cosmological sources, leads us to suggest theoretical studies
of gravitational radiation based on retrodiction (observation) rather than
prediction (experimentation).
