Papineau's Causal Argument for Materialism is Invalid

I’ve been thinking about Papineau’s causal argument for materialism again, since I’ve been putting together some lecture notes for a phil mind class I’m teaching. You can find the argument in chapter 1 of his book Thinking about Consciousness (Googlebooks preview here). Here’s how the argument goes:

(1) Conscious mental occurrences have physical effects.

(2) All physical effects are fully caused by purely physical prior histories.

(3) The physical effects of conscious causes aren't always overdetermined by distinct causes.

(C) Materialism is true

A couple of concerns occurred to me while thinking about this argument again. I’ll share one of those concerns in this post, and another one in a later post.

Here’s the first concern: is this argument even valid?

Papineau seems to think so. After laying out the premises, he says: “Materialism now follows.” I've read several responses to Papineau's argument, but I haven't found one questioning its validity. (If you know of one, please let me know!)

Let me flesh out my concern that this argument is not valid. It seems that we can describe a coherent view on which Papineau’s premises are true but his conclusion is false. I’ll first describe the view, and then I’ll explain why it isn’t a form of materialism.

Here’s the view. It’s pretty weird, but all I claim is that it is possible. Suppose nearly all conscious mental occurrences are irreducibly non-physical and non-functional, but one is not. The taste of a dirty papaya, let’s say, just is a brain state. That’s the only mental state that is also a physical state. All the other ones supervene on their corresponding physical states with at most nomic necessity. So this isn't a form of functionalism. (Like I said, it’s a weird view. But possible.)

Add to the view that all conscious mental occurrences have physical effects (so premise 1 is true). Add also that all physical effects have prior sufficient physical causes (so premise 2 is true). Therefore, on this weird view we’re considering, most of the time the physical effects of conscious causes are overdetermined. But not always: when the taste of a dirty papaya causes you to frown as you recall your youthful misadventures off Jamaica's beaten path, there is no overdetermination. On this view, the taste of a dirty papaya just is part of the physical cause of your frown. And so premise 3 is true on this weird view I’m describing.

Alright, so all the premises of Papineau’s argument come out as true on the view I’m describing. Now how about the conclusion? Would materialism be true on this weird view I’ve described? Insofar as we can have intuitions about semi-technical terms like “dualism” and “materialism,” I’d think it’s pretty clear that this view is not a form of materialism. It sure looks like a weird form of dualism.

But we need not rest our case on such intuitions. Though it's pretty hard to say exactly what materialism is, any adequate definition should at least entail this modal supervenience claim: the mental supervenes on the physical&functional with something stronger than nomic necessity. But that supervenience claim is false on this weird view. There are physically&functionally identical possible worlds that nevertheless differ with respect to mental facts. Sure, the facts about the taste of a dirty papaya can’t vary unless the physical facts vary, but that doesn’t hold with respect to all the other mental states on this view.

So, that’s the problem for Papineau. The premises of his causal argument could be true even while the conclusion is false, and so the argument is invalid.

Let me know if I’m wrong about this, but it looks like the argument would be valid were we to strengthen premise 3 to something like this: The physical effects of conscious causes are NEVER overdetermined by distinct causes. But that’s a substantially stronger claim than Papineau’s original premise 3, and no argument is given for this stronger version of premise 3. And while you might be repulsed by the suggestion of rampant overdetermination, it's more difficult to get worked up about the suggestion that it sometimes happens. That is, the stronger version of premise 3 has much less going for it.

So it looks like this particular argument for materialism doesn’t have teeth.