The overdetermination/epiphenomenalism dilemma for dualism

(1) Assume that there is mental-to-physical causation—that, for example, pains (mental causes) produce winces (physical effects)—but that mental events are distinct in nature from physical ones.

(2) Every physical effect has a wholly physical cause. (The causal-closure principle)

(3) Therefore either (a) every physical effect that is produced by a mental cause is overdetermined, i.e. wholly caused by some physical cause as well, or else (b) the mental is epiphenomenal with respect to the physical, i.e. there is no mental-to-physical causation.

Why a dilemma? Because the kind of rampant overdetermination required by (a) is improbable (Overdetermination occurs, but it is rare.) and epiphenomenalism seems to contradict the plain facts.