[István Aranyosi writes in with the following remarks...]
Karen Bennett has recently been working on showing that the exclusion problem cannot be escaped by the dualist. I think this is a quite widely held view nowadays. I have recently been working on showing the contrary: that it does not even arise for dualism, correctly understood. As Karen rightly points out [PDF], the exclusion problem is the fact that the following five propositions seem to be inconsistent, so one of them has to be denied:
1. DISTINCTNESS: mental events are distinct from physical events.
2. COMPLETENESS: every physical effect has a sufficient physical cause.
3. EFFICACY: mental events can cause physical events, and can do so in virtue of their mental properties.
4. NON-OVERDETERMINATION: effects of mental causes are not systematically overdetermined in a bad way (unlike the death of firing squad victims).
5. EXCLUSION: no effect has more than one sufficient cause unless it is overdetermined.
I argue in my paper [PDF] that one of the theses that a dualist should uphold is that behaviour itself is not purely physical, but it is ‘mentally enriched’, as I like to put it. The idea is not ad hoc: think of ‘my arm’s rising’ versus ‘my raising my arm’; the first is a purely physical effect, the second, according to me as a dualist, is both physical and mental. This yields a premise to be added to the five above:
6. Some effects of some mental causes are not purely physical.
I then try to show that adding 6 to 1-5 yields a consistent set of propositions. Take an effect e of a mental cause c, say ‘my raising my arm’ as caused by ‘my wanting to raise my arm’. The cause is both physical and mental: it’s both a wanting and a neural property. The effect is both physical and mental: it’s both a mere upward motion and an intentional raising. All this is compatible with:
1. Distinctness: wanting is a distinct property from the neural one.
2. Completeness: it holds for purely physical causes and a qualified version holds for not purely physical events: ‘the instantiation of the physical aspect of every effect has a sufficient physical cause’.
3. Efficacy: again it is interpreted as: events with mental aspects can cause events with physical aspects (and can do so partly in virtue of their mental aspect)
4. Non-overdetermination: the mental aspect of the cause is responsible for the mental aspect of the effect, mutatis mutandis for the physical aspect, so there’s no overdetermination
5. Exclusion: the effects of mental causes do not have either a physical or a mental sufficient cause; both aspects of the cause event jointly cause them.
Finally, let me clarify why I speak of some effects of some mental causes in (6). Paradigmatic mental causes are those like believing, wanting, desiring, etc. These are the ones for which (6) is true. Non-paradigmatic or derivatively mental causes are the ones that are effects of paradigmatic ones, for example ‘raising an arm’. Raising an arm can cause ‘destroying the spider’s web’, and this effect is purely physical even if caused by a mentally enriched cause.