Why can’t there be such a slide? The type-B materialist happily accepts that there are indeed phenomenal truths. However, “these truths concern an underlying physical reality” (14). Simply by saying that phenomenal truths concern an underlying reality isn’t the epistemic gap still left in place by the type-B materialist? (This sounds like non-reductive physicalism.)
Forget the epistemic gap.
Having an underlying physical reality doesn’t make phenomenal consciousness identical with such an underlying physical reality. Therefore the gap is still there. Is the gap also ontological in nature? Again, we need to explain this slide from the epistemic gap to the ontological gap. In addition, what does it mean to say that Mary “learns new facts in a new way” (14) when she leaves the room? This appears to be an argument that the facts are physical; though the experience of red doesn't add any new kind of facts. Thus there are no phenomenal facts or truths according to the type-B materialist.
What follows appears to be a statement that type-B materialists are in fact identity theorists. Chalmers writes that the
“most common form of type-B materialism holds that phenomenal states can be identified with certain physical or functional states”. (14)
How can the taste of milk or the experience of blue be identified with certain physical or functional states? Unless that identification doesn't mean:
the taste of mild = a certain physical or functional state.
It may simply mean that this identification amounts to the ‘underlying physical reality’ that subserves the taste of mild or the experience of blue. This, though, still leaves us with the epistemic gap, if not the ontological gap.
Now arguments by analogy reappear on the scene:
H2 O = water
DNA = genes
As we know, and as Saul Kripke has stressed, water is nothing above and beyond H2 O and genes are nothing above DNA. We can now say:
consciousness = functional/physical states, etc.
Can we slide from the functional and the physical to consciousness as we can slide from H2 O molecules to water and from DNA to genes?
Firstly we should say that the latter aren't “derived through conceptual analysis but are discovered empirically” (14). The concept [water] isn't the same as the concept [H2 O]. The concept [water] isn't ‘contained’ within the concept [H2 O] (to use Kantian terms). We discovered that water is H2 O and that genes are DNA. However, the former and latter concepts do have the same reference – they were “found to refer to the same thing in nature” (14). Does the concept [consciousness] refer to the same thing as the various physical and functional concepts? Doesn’t [consciousness] refer to, well, consciousness?
If the type-B materialist accepts the difference in concepts, though also the identity in reference, then he can admit to an epistemic gap; though also deny the ontological gap. This is conceptual pluralism and ontological monism in the manner of Spinoza and Donald Davidson’s ‘anomalous monism’. Again, can we say that the analogy between H2 O and water is the same as that between functional/physical states and conscious states?
Chalmers goes into greater detail as to why these analogies don't work.
Take the case of genes again. To explain genes “we merely have to explain why systems function a certain way in transmitting hereditary characteristics” (14). In other words, the explanation is functional/physical and it doesn't leave anything out. A functional/physical explanation of consciousness, on the other hand, would leave something out. The analogy, therefore, breaks down. The slide from DNA to genes and from H2 O to water can be explained in terms of deduction. Given
“a complete physical description of the world, Mary would be able to deduce all the relevant truths about water and about genes by deducing which systems have the appropriate structure and function”. (14)
Though is even this slide correct? It is indeed the case that water is nothing but H2 O and genes are nothing but DNA. However, can we really deduce water from H2 O and genes from DNA? No! That is why Kripke has eloquently argued that these identities are a posteriori in nature, not a priori. Surely this means that there's no deduction of water’s qualities, or water itself, from H2 O. Science had to discover this identity empirically, not through any kind of strict logical deduction.
Chalmers finishes off this Kripkean argument by saying that
“we cannot coherently conceive of a world physically identical to our own, in which there is no water, or in which there are no genes”. (14)
Again, what has psychological conceivability got to do with this? H2 O and water may be ontologically identical, and necessarily so; though what has this got to do with conceivability? The identity is a posteriori and scientific, not psychological. Indeed, as I think that Kripke himself has said, we may well be able to conceive of water, or water’s wateryness, etc., without thinking at all about it being a collection of H2 O molecules. In fact what if someone didn't know about this a posteriori identity? Clearly he can conceive of water without at the same time conceiving of H2 O.
We can say that this emphasis on conceivability appears to be dangerously Cartesian in nature - or perhaps even empiricist (in the sense that all imaginings must be based on prior ‘sense impressions’ and ‘ideas’, even if they are juxtaposed, inverted, etc.).
Chalmers then begins to cover issues which are more scientific in nature. The issue if the connection between the physical and consciousness.
We begin with fundamental laws of nature. Such things are deemed primitive. They can't be “deduced from more basic principles” (15). What has this to do with consciousness? Just as we have talked of the ‘primitiveness’ of the fundamental laws of nature, now we can talk of the “epistemically primitive connection between physical states and consciousness as a fundamental law” (15). This basically means that we can't explain that connection between the physical and consciousness. Perhaps there is literally nothing to explain because of its very basicness.
There can be a materialist position that accepts phenomenal states and consciousness generally. However, what they do require is that “physical states necessitate phenomenal states” (16). What this means is that “it is metaphysically impossible for the physical states to be present while the phenomenal states are absent or different”(16). This type-B materialist position is clearly a case of non-reductive materialism in that they allow phenomenal states and don't see them as identical to physical or functional states. More particular, like the supervenience theorist, the type-B materialist argues that if two physical states are identical, then their corresponding phenomenal states must also correspond or be identical. And just as we had epistemic entailment earlier on in the discussion, now we have physical, or ontological, entailment, in that
P – Q
must be necessary – physically and ontologically necessary.
Why is this entailment from P to Q ‘necessary’?
Here we must rely on Kripke again and his a posteriori necessary identities and truths. It is the case, according to Kripke, that “some truths are necessary without being a priori” (16). Traditionally, it was thought that all necessary truths can only be known a priori. In Kripke’s case,
“he argues that ‘water is H2 O’ is necessary – true in all possible worlds but not knowable a priori”. (16)
At every possible world, if there is a large collection of H2 O molecules there will be some water. Or, conversely, if there is some water there will be a large collection of H2 O molecules.
Now we have that epistemic gap again.
If we can only know the necessity of ‘water is H2 O’ a posteriori, that engenders an epistemic gap on our part from our knowledge of water to our knowledge of what makes up water. We can't know that ‘water is H2 O’ by a priori means. There is, again, an epistemic gap between water and H2 O.
necessarily water = H2 O
then there can't be an ontological gap because water just is H2 O!