  We discuss the use of dispersion relations for the evaluation of the
pseudoscalar contributions to the muon anomalous magnetic moment. We point out
that, in the absence of experimental data, reconstruction of light-by-light
scattering amplitudes from their absorptive parts is ambiguous and requires
additional theoretical input.
  The need for an additional input makes dispersive computations of the
hadronic light-by-light scattering contribution to g-2 akin to phenomenological
models, in spite of pretense to the contrary. In particular, we argue that the
recent proposal [1], based on the dispersive approach, satisfies short distance
constraints at the expense of unjustifiably large deviations from the chiral
limit.
