  Nuclear observables such as binding energies and cross sections can be
directly measured. Other physically useful quantities, such as spectroscopic
factors, are related to measured quantities by a convolution whose
decomposition is not unique. Can a framework for these nuclear structure
`non-observables' be formulated systematically so that they can be extracted
from experiment with known uncertainties and calculated with consistent theory?
Parton distribution functions in hadrons serve as an illustrative example of
how this can be done. A systematic framework is also needed to address
questions of interpretation, such as whether short-range correlations are
important for nuclear structure.
