PHILOSOPHY OF MIND
KRIPKE’S CONCEIVABILITY ARGUMENT
STUDY GUIDE
PROFESSOR JULIE YOO
Argument from Conceivability (aka “The Modal Argument”)1
Kripke’s argument is a modern re-formulation of Descartes’s Conceivability Argument. The
backdrop to Descartes’s argument is this:
1
Necessity and Contingency/a priori and a posteriori prior to Kripke
As Hume taught us, a posteriori claims always have the appearance of metaphysical contingency; that is, it seems
to us that the claims could have turned out to be false because epistemically they don’t present themselves to us as
metaphysically necessary. For instance, we discovered a posteriori that bread is nourishing to humans and that
grass is not nourishing to humans. It’s easy for us to coherently imagine that the facts of human nutrition could have
turned out to be otherwise, had the laws of human biology been different. That is why they seem to be
metaphysically contingent facts, and in this particular case, this appearance of contingency is a reality: bread will
still be bread even though it lacks the quality of being nourishing to humans, and grass will still be grass even
though it acquires the quality of being nourishing to human. All this is to say that being non-nourishing is a
contingent feature of bread and grass.
METAPHYSICS
EPISTEMOLOGY
SEMANTICS
NECESSITY: a necessary fact is a fact that
obtains in all worlds
CONTINGENCY: a contingent fact is a fact
that obtains in only some worlds
a priori KNOWLEDGE: the knowledge of a
fact independent to experience
a posteriori KNOWLEDGE: the knowledge of
a fact based on experience
ANALYTIC: a proposition is analytically true
when it is true solely in virtue of the meanings
of its terms
SYNTHETIC: a proposition is synthetically true
when it is true in virtue of the meanings of its
terms and the facts
This is quite different from claims such as “all triangles have three sides,” or “perfect roundness is circularity,”
whose truth we know a priori. With these claims, there is nothing contingent about them; they express facts that
are metaphysically necessary in that there is no possible world where some triangle has more or less than three sides
or where you have perfect roundness but lack circularity. Should one utter the statement, “some triangle has four
sides,” that person cannot be using the term “triangle” in the usual way. In short, being nourishing is a
metaphysically contingent property of bread; having three sides is a metaphysically necessary property of triangles.
Statements about contingent facts can will still make sense when negated; statements about necessary facts will
never make sense when negated. Philosophers rely on the following principles to gauge the metaphysical necessity
or contingency of a fact P:
P is metaphysically contingent if ~ P is coherently imaginable.
P is metaphysically necessary if ~P is not coherently imaginable.
Coherent imaginability includes i) honoring the truths of logic, ii) honoring the truths of mathematics, and iii)
honoring the facts about the meanings of terms.
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1. If the mind and the body are the same (are identical), then it is
not possible to have one without the other.
2. But it is possible to have one without the other.
3. Therefore, the mind and the body are not the same (dualism).2
(1) applies one of the basic assumptions we have of identity, and it is that if x really is y, then
they can’t come apart; if they can, then that means that they were never the same thing to begin
with. It is (2) that requires the brunt of the work. How does Descartes support (2)? With the
following argument:
1. I can coherently conceive that I have a do not have a body, but
I cannot coherently conceive that I do not have a mind.
2. If a situation S is coherently conceivable, then S is possible.
3. It is possible that I do not have a body, but it is not possible
that I do not have a mind.
(1) is supported by the Evil Genius Argument. In the Meditations, Descartes attempts to
establish that it is coherently conceivable that I do not have a body because it is coherently
conceivable that there is an Evil Genius who is tricking me into thinking that I have a body.
However, it is not coherently conceivable that I do not have a mind even if an Evil Genius is
trying to deceive me, since the very entertaining of the possibility that I am not thinking is itself
a testament to my thinking. (2) is a highly intriguing and critical premise of the argument. It is
in fact just a statement of the following principle:
Principle of Conceivability: If a situation S is coherently
conceivable, then S is possible.
The strength of this principle depends upon the plausibility of the standards for coherent
conceivability. It is no easy task specifying the standards, but to a first approximation, the
following guideline is a good start:
A situation S is coherently conceivable only if
a. S is consistent with the laws of logic,
b. S is consistent with the truths of mathematics,
c. S is consistent with the definitions of our terms.
Note that (a), (b), and (c) are only necessary conditions. The Principle of Possibility is highly
controversial, not only because it can be unclear what is coherently conceivable and what is not,
but also because it makes a suspect move from how we conceive of things, an epistemic fact, to
how things are in the world, a metaphysical fact.3
2
Descartes is not claiming that we do not have bodies, because he acknowledges that we clearly do. Nor is he
claiming that we can physically survive without our bodies (although he believed the Christian doctrine that we can
survive death). Rather, the argument is meant to establish that the mind is a logically separate component of a
person that can logically exist independently of her body.
3
Another way of running the argument is by using a certain principle about conceivability and essence. Let’s call
this Descartes’ Principle of Essentiality:
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Kripke’s Argument Against Materialism: The Master Argument
Kripke formulates the doctrine of physicalism in terms of properties, rather than facts, but like
Jackson, Kripke argues that mental properties (facts) do not lie within the domain of physical
properties (facts). Keep in mind that the term “physical” is a broad umbrella term that includes
not only “proper” physical properties (the microphysical and astrophysical properties studied by
physicists) but all the properties that supervene upon the “proper” physical properties. So this
would include special science properties (the properties studies by chemists, biologists, and so
on), the properties of neurophysiology (the reduction base favored by the Identity Theory), the
properties of behavioral dispositions (the reduction base favored by Behaviorism) and the
properties of causal/functional organization (the reduction base favored by Functionalism). The
basic idea behind Kripke’s argument is this:
A. If a mental property M is a physical property P (if M = P) then
it should not be possible for an individual to have M without P
or vice versa.
B. But it is possible for an individual to have M without P or vice
versa.
C. Therefore, no mental property M is a physical property P
(property dualism).
Kripke formulates the argument in terms of pain (the exemplar of a mental property M) and Cfiber stimulation (the exemplar of a physical property P), so his stated target is the Identity
Theory. But Kripke’s argument can be reformulated to attack any form of physicalism.
Before assessing each premise, let’s lay out some terminology. Identity claims, such as “Pain is
C-fiber stimulation,” are expressed using rigid designators. Rigid designators are terms that
pick out the same individual in every possible situation (“possible world”). Examples are proper
names, such as “Obama,” “Aristotle,” and “Paris.” Non-rigid designators are terms that pick out
different individuals in different possible situations (“possible worlds”). Examples include “The
current president of the US,” or “The youngest person in Sierra Tower at 7 pm April 2.” Proper
names are “rigid” in that they tag or refer to the same individual in different hypothetical
situations (also called “counterfactuals”). Imagine a hypothetical situation in which Bill Clinton
did not win any presidential elections. Still, the name “Obama” would still tag the same guy.
Descriptions like “the youngest person in Sierra Tower …” will pick out different individuals in
different hypothetical situations, depending upon who the individual happens to be who satisfies
the description. Kripke claims that terms like “pain” and “C-fiber stimulation” are like proper
names in that they function as rigid designators.
Descartes’ Principle of Essentiality: Some feature F is essential to an individual
x IFF it would be inconceivable for x to exist without F.
1.
2.
3.
4.
I can coherently conceive that I do not have a body.
No body is essential to me. (1, by the Principle of Conceivability)
I cannot coherently conceive that I do not have a mind.
My mind is essential to me. (3, by the Principle of Conceivability)
5. Therefore, my mind is not identical with any body. (2, 4, and the Law of
Identity)
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Rigid Designation and the Necessity of Identity
If two terms, T1 and T2, function as tags that always follow their objects (they are rigid
designators), and it is discovered that the terms refer to the same thing (individual or kind),
either by a priori reflection or by a posteriori observation, then the statement, “T1 = T2” is
necessarily true:
T1
T2
FIGURE 1
To say that “T1 = T2” is necessarily true is to say something with modal significance. It carries
the same modal force as the statement, “all triangles have three sides”: as long as the terms in
the statement retain their ordinary meanings and refer to the things we take them to refer to, there
is no possibility whatsoever under which the statement could be false. And the reason for this
has to do with the nature of identity: no thing is such that it can fail to be identical with itself.
Should the identity statement come apart, either through (re)reflection or further empirical study,
then that means that the two terms never referred to the same individual in the first place:
T1
T2
FIGURE 2
Premise (A) of Kripke’s master argument is supported by a highly intuitive principle about the
necessity of identity:
Necessity of Identity: If F = G, then it is not possible for you to
have F without G or vice versa.
Here’s the idea: if F and G really one and the same thing, then they cannot come apart. So
should they ever come apart, that means that F and G were never identical to begin with. The
Necessity of Identity is a principle about strict identity (as opposed to qualitative identity), and
the principle applies to both individuals (particulars) as well as properties (types). Our problem
concerns properties. So consider someone who said: “Roundness is the same property as
redness (roundness = redness).” We know that this claim is wrong, because there are numerous
cases where something is round but not red or red but not round. If they really were one and the
same feature, then they really couldn’t come apart like this (unlike roundness and circularity).
When making this judgment, we tacitly appeal to the Necessity of Identity. Here is another
example: “Being a sister is the same property as being a female sibling.” This is a true
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statement (if we limit the concept to biological sisterhood). By the principle of the Necessity of
Identity, there can never be a situation where an individual is one but not the other.
Kripke on Possibility and Conceivability
It is premise (B) of Kripke’s master argument that generates the controversy. Kripke argues that
it is possible for an individual to have a mental property (pain) without its corresponding
physical property (C-fiber stimulation, or any physical property), because we can coherently
conceive of the situation where the two come apart. The principle, which Kripke relies upon
here, is the Principle of Conceivability: If a situation S is coherently conceivable, then S is
possible.
Now, the physicalist can agree with everything stated so far – the Necessity of Identity, the
Principle of Conceivability, AND the coherent conceivability of pain without the occurrence of
C-fiber stimulation (or its converse). But the physicalist will argue that there are a number of
property identities whose falsity we can coherently conceive, but are still true in spite of our
conceptions. For instance, we can coherently conceive of water not being H20, or heat not being
mean kinetic energy. But, the physicalist argues, none of this means that these scenarios are
possible. Therefore, the physicalist continues, it is utterly unimpressive that we can conceive of
pain and C-fiber coming apart, because none of this has any consequences for the metaphysics of
what is possible.
We can draw upon the distinction between a priori knowledge and a posteriori knowledge to
support this point. Some identities are knowable a priori and others are knowable only a
posteriori. For example:
“Roundness = Circularity” is knowable a priori.
“Water = H2O” is knowable a posteriori.
Identities are necessary so this means that some necessities are a priori, other necessities are a
posteriori. Note that there is only one type of necessity here (what we can call logical necessity),
but two different ways of coming to know what is logically necessary. The crucial difference
between these two ways of coming discover these necessities lies with the apparent
conceivability of the falsity of the identity. Whereas a priori necessities cannot coherently be
conceived to be otherwise – you cannot coherently conceive of roundness occurring without
circularity or vice versa – a posteriori necessities can indeed appear to be coherently conceived
to be otherwise – there is the appearance that you can coherently conceive of water occurring
without H2O (after all, our world could have turned out to be like Twin Earth) and vice versa
(couldn’t H2O turn out to be alcohol rather than water?). But the crucial term is “appearance.”
According to Kripke, this is all that it is.
Kripke’s Conceivability Argument
Kripke’s conceivability argument consists of his reply to this objection, and it comes in two
parts. The first part consists of arguing that one really cannot coherently conceive of the falsity
of a true identity claim. That is, even though we go around saying things like “I can imagine
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water not being H2O,” it really makes no more sense than saying “I can imagine a round
square.” More generally, if p is necessary, then it is not coherently conceivable that not-p.
The second part consists of arguing that there is a crucial difference between scientific identities
like “water = H2O’ and “heat = mean kinetic energy,” on the one hand, and psychophysical
identities like “pain = C-fiber stimulation” on the other. Kripke thus rejects the objection that we
can run Kripke’s argument for other established scientific identities.
Part 1: In those cases where p is necessary and yet we appear to conceive of not-p, we are
actually mistaken about what we are imagining; we are not really thinking about p but rather
something closely resembling p.
Kripke’s Mistaken Conceivability Principle: If p is necessary, but
it appears that not-p, then there is an explanation of how one
mistakenly conceives that not-p; this explanation describes an
experience of something that looks and feels like p, but is not p. If
there isn’t such an explanation, then either p is not necessary or it
does not appear that not-p.
For example, when we say that we can imagine a scenario in which we have water but not H2O
(or the reverse), we are not, according to Kripke, really imagining a scenario with water in it, but
rather something that closely resembles water, but is not, in fact, really water. In other words,
the scenario we are imagining is a scenario where there is a substance that appears and feels to us
just like water, but is not in fact water. It is “fools-water.” To make this more vivid, consider
how we can easily be fooled by fools-gold. Fools-gold gives us (non-professional jewelers) the
sensations we have when we experience real gold; but fools-gold is not real gold.
Part 2: The Mistaken Conceivability Principle applies to scientific identities, Kripke claims, but
it does not apply to pain and C-fiber stimulation (and, by extension, to any identity between a
mental and a physical property). Try running the principle for pain. When it appears to you that
pain can occur without C-fiber stimulation and vice versa, this appearance cannot be explained
away by appealing to an experience of something that looks and feels like pain, but is not pain.
This is absurd, since anything that looks and feels like pain just is pain. In other words, there
cannot be anything like “fools-pain,” the way there can be fools-X for any X that appears in a
scientific identity, because there is not appearance-reality distinction for sensations the way there
is the distinction for non-subjective, scientific, identities.
The Case of Heat and the Case of Pain
If we apply the above principles about rigid designation and the necessity of identity, we can see
that:
•
•
Figure 1 describes the case of heat and mean kinetic energy.
Figure 2 describes the case of pain and c-fiber stimulation.
Let’s first go through the case of heat and mean kinetic energy. What is heat? Heat is mean
kinetic energy. That is what we discovered when we scientifically studied the phenomenon over
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many many centuries. Since this is something we discovered, we came to know that “heat is
mean kinetic energy” is true a posteriori, and because it is not a priori, it has the appearance of
contingency. That is why it seems we can say:
“Heat is not mean kinetic energy.”
But when we say this, we are saying something as false as:
“A triangle is not a three-sided figure.”
If we had to characterize what is going on in the mind of someone who utters the second sentence,
we would have to do it this way:
$*#$*!!*# is not a
three-sided figure.
“A triangle is not a three-sided figure.”
We know that it is impossible for a triangle not to have three sides. Because we know this a
priori, it is fairly easy for us to see that the $*#$*!!*# has to represent a non-triangle in order to
make sense of her utterance. In the case of true necessary a posteriori statements, Kripke
advises us to give a similar diagnosis:
@%^^!@ is not
mean kinetic energy
“Heat is not mean kinetic energy.”
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As in the case of $*#$*!!*# above, @%^^!@ has to represent non-heat in order to make sense of
her utterance. But unlike the case above, because we do not have an a priori grasp of the nature
of heat, this is not as easy to see. However, on Kripke’s view, this is exactly what is going on.
In particular, @%^^!@ is something that feels similar to heat to the speaker in that @%^^!@
causes the speaker to sweat, want to drink water, etc., but just as “fools-gold” has all the
superficial appearances of gold, @%^^!@ has all the superficial appearances of heat. It is
“fools-heat.” The final result is that because we can explain away the apparent contingency of
heat being mean kinetic energy, we can maintain that it is necessarily true. In other words,
“heat” and “mean kinetic energy” refer to the same phenomenon, not to different phenomena.
Kripke’s anti-materialist argument turns on the claim that no comparable diagnosis can be given
of a claim that identifies a conscious state, like pain, with any physical state, like c-fiber
stimulation:
asdkfjh is not
c-fiber stimulation.
“Pain is not c-fiber stimulation.”
In the case of this statement, its denial cannot be explained away. It makes no sense to say that
asdkfjh feels just like pain, but is not pain, for the fundamental reason that anything
that feels like pain just is pain. Therefore, the terms “pain” and “c-fiber stimulation” do not refer
to the same phenomenon after all. They refer to distinct kinds of things, things that in our world,
happen to co-occur, just has hearts and kidneys co-occur in animals, but mere co-occurrence is
not identity. The fact that hearts and kidneys co-occur is a contingent fact, and so is the fact that
pain occurs with c-fiber stimulation.
Questions
1. Why can there be such things as fools-gold or even fools-water, but not fools-pain?
2. How does Kripke’s argument help to support Nagel’s claim that consciousness is not
reducible?
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