Malcolm on Mysterianism

And so it is with the mysterian materialist. He bids me accept propositions that as far as I can tell are not propositions at all. A proposition is a sense, but the 'propositions' he bids me accept make no sense. For example, he wants me to accept that my present memories of Boston are all identical to states of my brain. That makes no sense. Memory states are intentional states: they have content. No physical state has content. So no intentional state could be a physical state. The very idea is unintelligible. Where there are no thoughts one can always mouth words. So one can mouth the words, 'Memories are in the head' or 'Thoughts are literally brain states.' But one cannot attach a noncontradictory thought to the words.

You are right that the mysterian materialist will maintain that some physical states do have content.  But he also maintains that we will never be able to understand how this is possible.  Thus your 'not yet understand' is not accurate. As Colin McGinn, head honcho of the mysterian materialists, puts it, "My thesis is that consciousness depends on an unknowable natural property of the brain." (The Mysterious Flame, p. 28,emphasis added)  Someone who holds that with the advance of neuroscience we will eventually solve the mind-body problem is not a mysterian.

The mysterian materialist position is that mental activity just is brain activity.  If that is actually so, then it is possibly so whether or not we can render intelligible to ourselves how it is so.  For McGinn, we will never render this intelligible because it is impossible to do so.  The mind-body problem is "perfectly genuine" (212) but has never been solved and is indeed insoluble because "our minds are not equipped to solve it, rather as the cat's mind is not up to discovering relativity theory or evolution by natural selection." (212)

You  are right: my impossibility is his actuality.  For him, the proposition that some physical states have content is true but a mystery.  So he asserts what he takes to be a well-defined and possibly true proposition — *Some physical states have content* — but also asserts that the question of how this proposition is possible will not ever, and cannot ever, be answered due to the limitations of our cognitive architecture.

My claim is that there is no well-defined proposition before us, or rather that there is no proposition before us that could be true.  There is the sentence 'Some physical states have content' but this sentence expresses no proposition  that could be true.  It's a little like 'Some color is a sound.'  That sentence does not express a proposition that could be true.  I don't believe you would credit the sort of  mysterian who maintains that it is true that some colors are sounds, and therefore possibly true, despite our inability to explain how it is true.  You would laugh out of the room the guy who said it was true but a mystery.  You would say, 'Get out of here, you are talking nonsense.'

How do we know it is nonsense?  We know this by thinking attentively about colors and sounds and by grasping that a color is not the sort of item that could be a sound.  Similalrly, we know it is nonsense to identify a memory of Boston with a brain state by thinking attentively of both and grasping that the one is not the sort of item that could be identical to the other.  (Because the one has content while the other doesn't so the two cannot be identical by the Indiscernibility of Identicals.)

Moving from content to qualia, I would say 'This smell of burnt garlic is identical to some brain state of mine' is on all fours with 'Quadruplicity drinks procrastination.'  It can't be so, and for a very deep reason: the very electro-chemical and other vocabulary (axons, dendrites, synapses, diffusion of sodium ions, voltage differentials, etc.) cannot be meaningfully combined with the vocabulary of phenomenology..  When you combine them you get nonsense.  The resulting propositions — if you want to call them that — cannot be true.

Isn't "No physical state has content", in this context at least, question-begging?

I don't believe I am simply begging the question.  It is more complicated than that.  It may help if I lay out both the mysterian and my argument.

Mysterian Argument

1. Mental activity is just brain activity. (Naturalist assumption)
2. We cannot understand how mental states could be identical to brain states.
Therefore
3. This inability to understand does not reflect an objective impossibility but an irremediable limitation in our cognitive architecture:  our minds are so structured that we will never be able to understand the mind-body link.

My Argument

2. We cannot understand how mental states could be identical to brain states.
~3. This inability reflects an objective impossibility.
Therefore
~1. Mental activity is not just brain activity.

The deep underlying issue here seems to be this:  Is our inability to understand how such-and-such is broadly-logically possible a sufficient reason for denying that such-and-such is objectively broadly-logically possible? To put it another way, the issue is whether there could be true mysteries, where a mystery is a proposition that by our best lights must appear either to be or to entail a broadly-logical contradiction.

This issue lies deeper than the naturalism issue.