Monroe wrote:
It has become popular among Quine admirers, "naturalists", straight up Dennett fans, and others to denegrate the validity or usefulness of thought experiments altogether in order to attack the standard arguments against materialism. While some critiques of the use of thought experiments might have a lot of wisdom to them, I contend here that the questions involved in the mind/body problem are such that thought experiments are a necessary part of addressing the issue.

Whatever your particular formulation of the mind/body problem, everyone seems to agree that materialism is false if Chalmersian zombies are possible. And no one seriously believes that zombies are actual. So the mind/body problem is not a question of whether there will ever be any actual instances of normally functioning brains without minds, but whether brains make minds necessary. Thus the question itself goes beyond mere empirical facts into questions of possibility and necessity.

Now, it is all too easy to offer something that looks like a logical proof that brains do not imply consciousness. The vocabulary and sentences used to describe consciousness are very different from the terms of physics, chemistry, and neuroscience. We can simply assign truth values to sentences of one vocabulary independently of sentences of the other vocabulary. One of these assignments will thus be a world of zombies-- We assign "true" to the existence of brains as they are described in this world, and "false" to the existence of consciousness.

So in order for there to be any ground for the materialist to stand on, there must be some deeper constraints between the two vocabularies in order to render zombies impossible. But since empirical facts will not have enough bearing on this question of possibility, we must turn to the only thing left: thought experiments, or conceptual analysis.

The task of the materialist is thus to show that our concepts of mind are intimately connected with the concepts in physical science, establishing logical constraints between the vocabularies which preclude zombies and make it that brains imply consciousness.

On the other hand, if we can come up with coherent thought experiments in which consciousness and brain activity do not coincide in the same way they do in this world, then materialism must be mistaken. It seems to me that this has already been done. It is thus the task of the materialist to somehow argue that a fundamental misunderstanding has taken place in these analyses.

Now, it is all too easy to offer something that looks like a logical proof that brains do not imply consciousness. The vocabulary and sentences used to describe consciousness are very different from the terms of physics, chemistry, and neuroscience. We can simply assign truth values to sentences of one vocabulary independently of sentences of the other vocabulary. One of these assignments will thus be a world of zombies-- We assign "true" to the existence of brains as they are described in this world, and "false" to the existence of consciousness.

So in order for there to be any ground for the materialist to stand on, there must be some deeper constraints between the two vocabularies in order to render zombies impossible.

But since empirical facts will not have enough bearing on this question of possibility, we must turn to the only thing left: thought experiments, or conceptual analysis.

The task of the materialist is thus to show that our concepts of mind are intimately connected with the concepts in physical science, establishing logical constraints between the vocabularies which preclude zombies and make it that brains imply consciousness.

Are saying that given the current evidence, consciousness is not implied by what we know of brain activity?

That in our current best theories of brains, zombies are possible?

A contradiction is asserting the affirmation and negation of a proposition. If the two vocabularies contain none of the same sentences, then no sentence will be both affirmed and denied by assigning truth values to the different vocabularies independently.

Let me clarify: By brain activity entailing consciousness, I do not mean the set of all sentences involving some reference to brain activity entailing the set of sentences about consciousness. There are empirical facts about the correlations between consciousness and brain activity. These facts involve reference to brain activity, so they obviously can be used to derive the presence of conscious phenomena from the presence of brain activity. What I mean by brain activity entailing consciousness is the set of sentences involving reference only to brain activity entailing consciousness. Gathering empirical correlations will not help to establish the entailment, since the correlations will use words from both vocabularies.

And so, therefore, the existence of brains that lack consciousness is not contradicted by our best current theories of brain activity (it is obviously contradicted by what we know about consciousness, but that is not the question).

And thus, in our best current theories of brain activity, zombies are possible.