  While the axion was originally introduced to wash out CP violation from
strong interactions, new sources of CP violation beyond QCD might manifest
themselves via a tiny scalar axion-nucleon component. The latter can be
experimentally probed in axion-mediated force experiments, as suggested long
ago by J.E. Moody and F. Wilczek. In the present note, I review the physical
origin of CP-violating axion couplings and point out the special role of the
QCD axion as a low-energy portal to high-energy sources of CP violation.
