It has been suggested that the phenomenological proton-proton spin-orbit force arises as a `Thomas term' in the Schroedinger equation, associated with the repulsive core. Since the potentials involved are very strong, the usual derivations are inadequate. Here the problem is formulated as the scattering of two Dirac particles interacting via a truncated potential which couples only positive energy states. For mathematical simplicity a one-body problem is introduced. An iterative method of solution is proposed and carried out to second order in the case of a square repulsive core potential. The method is found to converge satisfactorily. The phase shifts show splitting of the type expected from the Thomas term argument.