With the two premises now in place, the Problem of Interaction in our final version is straightforward. Assume for reductio that our souls routinely cause behavior. By Completeness, such effects also have sufficient physical causes, so behavior is systematically overdetermined. But this contradicts No Overdetermination. The dualist’s options would then seem to be severely limited. One is to embrace epiphenomenalism, a doctrine on which the mental, while caused by the physical, exerts no “downward” causal influence in return. A more radical option, parallelism, depicts bodies and souls as running in tandem, with no causal influence in either direction.