  In this paper, which is of programmatic rather than quantitative nature, we
aim to further delineate and sharpen the future potential of the LISA mission
in the area of fundamental physics. Given the very broad range of topics that
might be relevant to LISA, we present here a sample of what we view as
particularly promising directions, based in part on the current research
interests of the LISA scientific community in the area of fundamental physics.
We organize these directions through a "science-first" approach that allows us
to classify how LISA data can inform theoretical physics in a variety of areas.
For each of these theoretical physics classes, we identify the sources that are
currently expected to provide the principal contribution to our knowledge, and
the areas that need further development. The classification presented here
should not be thought of as cast in stone, but rather as a fluid framework that
is amenable to change with the flow of new insights in theoretical physics.
