  For a relativistic particle moving in the presence of mean scalar and vector
fields, the energy at second order in the scalar field is shown to contain two
contributions in general. One is a momentum-dependent repulsive interaction
satisfying a low-energy theorem pointed out by Wallace, Gross and Tjon. The
other does not vanish at zero-momentum and involves a ``polarisability" of the
particle by the scalar field. The first of these contributions is independent
of the details of the structure of the particle and the couplings of its
constituents to the external fields. The appearance of such a piece in the
central nucleon-nucleus potential thus would support the existence of strong
scalar fields in nuclei, without requiring the use of a Dirac equation for the
nucleon.
