  Although the exact Bethe-Salpeter equation is certainly the appropriate
field-theoretic framework to describe the non-perturbative problem of
scattering and bound states, the inevitable truncations introduce
inconsistencies such as loss of symmetries or incorrect one-body limit. I
conjecture that these problem can be overcome if the truncation preserves the
field-redefinition invariance of the exact equation. A sum rule for
light-by-light scattering can provide a testing ground of this conjecture.
