The Identity Theory in 2-D
Twenty years ago the type-type mind-brain identity theory looked dead in
the water. It had suffered tremendous blows from the arguments of Putnam
(Putnam 1967), Kripke (Kripke 1980), and Fodor (Fodor 1974). From our vantage
point now we can see that things are not as bad as they first appeared and the
identity theory has enjoyed a surge of interest in the wake of recent work by people
like Chris Hill (1991), John Perry (Perry 2001), Tom Polger (Polger forthcoming;
2004), and Ned Block (Block 2009). Within the identity camp there are two broad
traditions that roughly correspond to whether one follows Smart/Place or
Armstrong/Lewis. The main difference concerns how we will eventually come to
discover the mind-brain identities and whether these identities are in principle
knowable a priori.
On the Place/Smart view the mind-brain identities are postulated because
they offer the most parsimonious ultimate theory. The identities are truly a
posteriori in the sense that they cannot be known to be true a priori even in
principle. On the broadly Lewisian view the identities are entailed by the theories
that make them true. These identities could be known a priori, at least in principle.
This is well known. What is less well known, perhaps, is the kind of identity theory
that one gets if one takes a broadly Lewsian/Armstronian approach updated in light
of the advances in two dimensional semantics. . In this paper I will briefly rehearse
the two-dimensional view of David Chalmers (Chalmers 2008) and then formulate
the identity theory in its terms. I will conclude by showing how the 2-D identity
theory is immune to most common criticisms of the identity theory.
I. The 2-D Framework
In my view two-dimensional semantics is motivated by the intuitive idea that
there are two different ways of picking things out in the world. One way of picking
things out is via a description. To pick something out that way is to find the thing in
the world that “fits” the description. The second way of picking something out is by
pointing at it. “That stuff,” one says pointing at some liquid in a glass, “what if that
stuff had been XYZ” is a very different question from “what if the stuff that falls from
the skies, fills our lacks, etc, had been XYZ”. As David Chalmers has pointed out this
intuitive distinction corresponds very well with two ways of thinking about
necessity and possibility. The most familiar way of thinking about these is counterfactually. When one does this one takes the actual world as fixed, or given, and
reasons from there. The other is what David Chalmers calls indicatively. This is less
familiar but is equally intuitive. When we think about things this way we do not
anchor ourselves to the actual world. We think about each possible scenario not as
something that is counter-factual but rather as though it were the actual world.
What we get then is a unified space of possibilities that can be thought of in two
different ways each of which corresponds to one of the ways we have of picking
things out in the actual world. From this intuitive starting point we can add some
technical terms to keep things straight. Before doing so I should just note that one
need not endorse the whole program of the 2-D movement to see that it is useful to
think about the space of possibilities.
We can start by introducing the primary and secondary intensions of a
statement. Let’s take our old standby ‘water is H2O’ and our two ways of
considering possible worlds as either actual or counter factual. So, ‘water is H2O’ is
true when we consider any possible world as counter-factual. This is because ’water
is H2O’ is an a posteriori necessity. There are no worlds, considered as counterfactual, where water isn’t H2O. This we call the secondary intension of the
statement. It is evaluated counter-factually and so is true even at worlds like Twin
Earth because there is no water on Twin Earth considered counter-factually. But if
we consider Twin Earth as actual instead of counter factual, ‘water is H2O’ comes
out false because on Twin Earth ‘water is XYZ’ is true and so when we consider Twin
Earth as actual ‘water is H2O’ is false. That is to say that if Twin Earth were the
actual world ‘water is not H2O’ would be true (because ‘water is XYZ’ is true there).
The primary intension of a statement is given by asking whether it is true or
false at possible worlds considered as actual while the secondary intension of a
statement is given by asking whether it is true or false at possible worlds considered
as counter factual. In effect then the primary intension of a statement is given by
some kind of reference fixing description and then we determine whether the
statement is true or not by taking a possible world and letting the description fix the
reference at that world and the secondary intension of a statement is given by
assigning the actual reference of the terms in question and holding that fixed as we
vary our counter factual worlds.
Primary and secondary conceivability are then defined in terms of the kind of
intension at work. So, take ‘water is not H20′. Chalmers accepts that there is a sense
in which this is not conceivable. This is the sense in which we give ‘water’ the
reference that it actually has. Then ‘water is not H20′ is equivalent to saying ‘H20 is
not H20′ which is a contradiction. This is to use secondary conceivability. ‘Water is
not H2O’ is still primarily conceivable since if we consider Twin Earth as actual it
will be true. This is because we assign ‘water’ just the reference fixing description
and so to say that water is not H2O is just to say that ‘the stuff picked out in the
same way we pick out H2O is not H2O’ which is true at Twin Earth. This captures
Kripke’s way of putting it. When we think that we are conceiving of water not being
H2O we are really conceiving of a person who is in the same epistemic situation as
we are when we pick out H2O but isn’t picking that stuff out (i.e. Twin Earth).
II. Applying the Framework
If we take this and combine it with Lewis/Armstrong identity theory we end
up with the claim that the primary intension of psychological predicates just are the
causal-role definitions of those terms. So the primary intension for ‘pain’ will be ‘the
state caused by … and which causes …’ as well as all of the folk-psychological
platitudes about pain. This primary intension will be used to pick out the painful
stuff across possible worlds. One of the central claims of the identity theory is that
mind-brain identities are no different than ordinary scientific identities like
water=H2O. As we have just seen in the 2-D framework these kinds of statements
have a necessary secondary intension and a contingent primary intension. If we
read the statement as, in effect, ‘H2O is H2O’ then it is necessary as all worlds where
there is H2O are worlds where there is H2O. But the primary intension is
contingent. There is a possible world, which had it been the actual world it would
have been the case that water was not H2O. This is just Twin Earth. If mind-brain
identities are just like ordinary scientific identities then they will have a necessary
secondary intension and a contingent primary intension.
That means that our description, “the painful stuff” will pick out a certain
brain state at the actual world if the identity theory is true. Kripke taught us that
these identities were necessary but 2-D semantics taught us that this means they are
secondarily necessary and primarily contingent. So if we take the actual world as
given and consider other possible worlds as counter-factual then all worlds where
there are pains are ones where it is a brain state because it is (ex hypothesi) true at
the actual world. However if we take the primary intension we will find possible
worlds where brain states are not picked out. But just as Twin Earth is no threat to
physicalism about water neither are these dualist worlds threats to physicalism
about consciousness. This shows that Cartesian thought experiments about ghosts
are no threat to the identity theory in 2-D. Ghost worlds are just the ones where the
primary intension of “the painful stuff is physical’ is false which is precisely what
makes it contingent and this is exactly what we expect.
What about zombies? According to the identity theory in 2-D the zombie
world is akin to a world that is physically identical to our world in that it has H2O
but is stipulated to lack water. This is not even conceivable on the 2-D view. Given
what we know now we can see that we can in fact deduce water facts from H2O facts
and that shows us that there are no possible worlds like the one described. Just
given the H2O facts alone necessitates water facts. If the identity theory is true then
the same is the case for mind-brain identities. The zombie world is then
inconceivable. What are we to say to the charge that the zombie world seems
conceivable? Is this an objection? No. The problem is that it is equally conceivable
that consciousness be a physical property. I have previously (Brown 2010) called
these creatures ‘shombies’. Shombies are creatures that are completely and
exhaustively physical but that are conscious in exactly the same way that I am.
Below I will briefly sketch a way of conceiving of shombies.
Suppose for the sake of argument that the Higher-Order thought theory of
consciousness as defended by someone like David Rosenthal (Rosenthal 2005) turns
out to be correct. On this view having a conscious pain consists in one having a
occurent seemingly non-inferential thought to the effect that one is oneself having a
pain. That is, one is conscious of oneself as being in pain and this results in its being
painful for one. Having this thought can then be identified with having a certain
brain state. In that case we could in principle deduce the identity of pain and the
brain state using the higher-order theory of consciousness. This is, of course, highly
controversial but we can give an argument for it. It is, roughly, the argument that
Rosenthal himself gives in many places. Take the case of listening to an orchestra. If
one has no concept of what a bass clarinet is one will not consciously experience the
sound of the bass clarinet as such, though one’s experience of it may be conscious in
some other respect (that is to say one will have the relevant first-order states with
their qualitative characters and perhaps even higher-order thoughts about them but
not as having bass-clarinet* qualities). Once one acquires the concept ‘bass clarinet’
one’s experience is different in a phenomenological way. What it is like for one to
hear the orchestra will differ in precisely the sense that it will now sound like there
is a bass clarinet in the orchestra to one. The same case can be made for wine
What cases like this give us is data that learning a new results in new
conscious phenomenology. This suggests that the precise content of the HOT
determines what it is like for one and this very fact is what is supposed to aid in our
explanation of phenomenal consciousness. At the very least these kinds of data open
up the space of possibilities and suggest that it could be the case that having a HOT
accounts for phenomenal consciousness. In response to the objection that one may
grant that applying concepts results in a change in one’s phenomenology but may
still deny that HOTs can explain the existence of phenomenology in the first place.2
Rosenthal says,
Suppose I am in pain and that pain is conscious, but I am not aware of
whether the pain is throbbing or dull or sharp. Because the pain is conscious
there is something that it is like to be in it...suppose, now, there is some
physiological reason to think that the pain is throbbing, as opposed to dull or
sharp. So what it is like to be in this pain leaves out one of its qualitative
properties: that of being a throbbing pain and if qualitative states can occur
without one’s being conscious of all their mental qualities, what reason can
we give to deny that such states can occur without our being aware of any of
those qualities? (p. 155)
When one subtracts out the content ‘throbbing’ from the HOT one’s conscious
phenomenology lacks this aspect. If we were to add it back in one’s conscious
phenomenology would again include it. This is because “what it’s like for us to be in
a specific qualitative state is determined by the way consciousness represents that the content of the HOT,” (p.173). If we imagine again abstracting away the
Rosenthal often does make the case in terms of wine tasting, see for instance p. 1887-188
One could do this, for instance, by claiming that applying the concepts has a causal effect on the first
order experiences while denying that phenomenology is constituted by conceptual application.
content of the HOT so that one was conscious of the pain just as some sensation or
other the experience would be correspondingly more generic for the creature that
had it.3 If, as this subtraction process seems to support, peeling of the final concept
strips the experience of any phenomenal consciousness then it is not unreasonable
to suggest that it is the having of the higher-order state that constitutes the
phenomenology. Notice that this subtraction argument is not supposed to show that
applying concepts constitutes phenomenology but rather only to show that this is a
plausible way to interpret the data of wine tasting and like phenomena. There may
be other theoretical accounts but the higher-order view remains a live option and all
we need for the present purposes is that the scenario that I just sketched is
conceivable. This certainly seems conceivable without any obvious contradiction.
What about multiple realizability arguments? Let us discuss the science
fiction example of Commander Data from the Star Trek series. Data is portrayed as
having a ‘positronic’ brain, which is supposed to be something like a functional
isomorph of the human brain. If we consider the world that Data is in as the actual
world then we are thinking about a world where functionalism is true. But if the
identity theory is true at the actual world and we consider the Data world counterfactually we consider a world where there are no mental states. Considered counterfactually a mental state must be a brain state and since Data has no brain states he
has no mental states. What this means is that the actual world cannot be one where
there are mental states that are multiply realized. This is the respect in which the 2D identity theory most dramatically differs from Lewis’ approach. Lewis would
Rosenthal makes this subtraction argument in “Explaining Consciousness” page 415, which appears
in the Chalmers anthology.
argue that these identities are secondarily contingent but the 2-D view incorporates
Kripke’s insights and treats them as secondarily necessary. Is this an objection? That
is, is there any reason to think that functionalism is actually true?
Polger (Polger forthcoming) has recently argued that we do not have much
reason to think that mental states are actually multiply realized. The only real
evidence we have for multiple realization comes from intuitions about what is
possible. But if the identity theory is true of the actual world then what these
intuitions are really about are various possible worlds that are not candidates for
being the actual world. Instead what one imagines is that there were creatures that
were in the same epistemic situation that we are in and were not in the
corresponding brain states. That is, we conceive of the world that makes the
primary intension true just like we did with H20. When we conceive of Twin Earth
we conceive of a world that makes the primary intension of ‘water is H2O’ come out
false (because at Twin earth considered as actual water is XYZ). Notice that this is
not enough to threaten physicalism. No one thinks that physicalism about water is
threatened by twin Earth why should physicalism about pain be threatened by the
Commander Data world? Of course if it could be shown that Data is secondarily
possible then we have a problem. However, as I have previously argued, if we see
brain states as patterns of synchronous neural firing (Brown 2006) we can
accommodate some variation in the underlying realizers without threatening the
identity theory. As Polger says, we identify mental states with brain states not with
molecular processes that occur in brains.
In closing let us look at a more recent objection. In a recent paper Ned Block
(Block forthcoming) targets the Lewisian view in favor of the Place/Smart view and
argues that the Lewis style view is incompatible with the metaphysics of
physicalism. Block distinguishes between ontology and metaphysics. Ontological
physicalism is just the claim that in our ontological commitment to the existence of
qualia we commit ourselves only to physical entities (ontological dualists deny this).
Metaphysical physicalism is the claim that qualitative properties are essentially or
metaphysically physical. That is to say that all qualitative properties will share the
same physical properties in so far as they are physical. The Lewis style physicalism
is ontologically but not metaphysically physicalist. This is because as it happens all
of the realizers of mental states are physical but metaphysically pain is a functional
state for Lewis and only contingently a physical state. Metaphysical physicalism –
real physicalism in Block’s view– says that it is not contingent but necessary that
pain is a physical state.
But if we adopt the 2-D framework and put the Lewisian claims in terms of it
this is no longer a problem. On this kind of view the functional definition gives us
the primary intension of ‘pain’ and the physical state gives us the secondary
intension. This allows us to treat ‘pain’ just as we do ‘water’. ‘Water is H2O’ has a
contingent primary intension and a necessary secondary intension. So we can
update Lewis view that ‘pain’ isn’t a rigid designator as the claim that the primary
intension of pain is contingent (just like ‘water’). ‘Pain’ is still a rigid designator in
the ordinary sense that its secondary intension is necessary. In all worlds
considered as counter-factual pain is a brain state. However we accommodate the
conceivability of Martians and disembodied minds by noting that in some worlds
considered as actual pain is not a brain state (just as in some worlds considered as
actual water is not H2O). This does not threaten the identity; it is the usual way that
theoretical identities work. Notice also that this 2-D identity theory is a
metaphysical physicalism in Block’s sense and not merely an ontological
physicalism since it holds that the mind-brain identities have necessary secondary
Work Cited:
Block, N. (forthcoming) “Functional Reduction” in D. Sosa, T. Horgan and M. Sabatés
(eds) Supervenience in Mind: A Festschrift for Jaegwon Kim. Cambridge, Mass.: MIT
Block, N. (2009) “Comparing the Major Theories of Consciousness,” in Michael
Gazzaniga (ed.) The Cognitive Neurosciences IV. MIT Press
Brown, R. (2006). “What is a Brain State?” Philosophical Psychology 19 (6): 729-742
Brown, R. (2010) “Deprioritizing the A Priori Arguments against Physicalism”
Journal of Consciousness Studies 17 (3-4): 47-69
Chalmers, D. (2008) “Two-Dimensional Semantics” in E. Lepore & B. Smith, (eds)
Oxford Handbook of the Philosophy of Language. Oxford University Press.
Fodor, J. A. (1974). "Special Sciences (Or: The Disunity of Science as a Working
Hypothesis)." Synthese 28: 97-115.
Hill, C.S. (1991). Sensations: A Defense of Type Materialism. Cambridge: Cambridge
University Press.
Kripke, S. A. (1980). Naming and Necessity. Cambridge, Harvard Univ Pr.
Perry, J. (2001) Knowledge, Possibility, and Consciousness MA: MIT Press.
Polger, T. (forthcoming) “Are Sensations Still Brain Processes?” Philosophical
Polger, T. (2004). Natural Minds. Cambridge, MA: MIT Press.
Putnam, H. “The Nature of Mental States” in Rosenthal, D. (ed) The Nature of Mind
Rosenthal, D. (2005) Consciousness and Mind OUP

The Identity Theory in 2-D