  The spectroscopic factor has long played a central role in nuclear reaction
theory. However, it is not an observable. Consequently it is of minimal use as
a meeting point between theory and experiment. In this paper the nature of the
problem is explored. At the many-body level, unitary transformations are
constructed that vary the spectroscopic factors over the full range of allowed
values. At the phenomenological level, field redefinitions play a similar role
and the spectroscopic factor extracted from experiment depend more on the
assumed energy dependence of the potentials than on the measured
cross-sections. The consistency conditions, gauge invariance and Wegmann's
theorem play a large role in these considerations.
