Locke's Exclusion Argument

by Walter Ott

1. The Exclusion Problem

Jaegwon Kim's exclusion argument threatens to make mental states and events epiphenomenal. Take physical event P, say, the stimulation of a certain set of neurons, and mental event M, the volition to move one's hand. Now consider P*, the movement of one's hand. Both P and M compete for the title "cause of P*." But assuming the closure of the physical world and the impossibility of overdetermination, P must win. P* has a complete and sufficient physical cause; in our example, this either is or includes P. (It might also include other physical background conditions.) So what work is left for M to do? M seems to be a mere side effect of P, with no causal powers of its own. And even if we allow causal overdetermination, something of the same problem remains: the physical world would still go its own way, even if mental events were removed.

This argument at most threatens those materialist views that take the relation between M and P to be one of supervenience. An identity theorist has no problem here, since M and P just are the same event, described in different ways. And a dualist (whether property- or substance-) might be willing to bite one of the many bullets on offer, by, say, denying that P* has a sufficient physical cause.