Thagard (2012) contains a nicely compact passage on thought experiments:
Grisdale’s (2010) discussion of modern conceptions of water refutes a highly influential thought experiment that the meaning of water is largely a matter of reference to the world rather than mental representation. Putnam (1975) invited people to consider a planet, Twin Earth, that is a near duplicate of our own. The only difference is that on Twin Earth water is a more complicated substance XYZ rather than H2O. Water on Twin Earth is imagined to be indistinguishable from H2O, so people have the same mental representation of it. Nevertheless, according to Putnam, the meaning of the concept water on Twin Earth is different because it refers to XYZ rather than H2O. Putnam’s famous conclusion is that “meaning just ain’t in the head.”
The apparent conceivability of Twin Earth as identical to Earth except for the different constitution of water depends on ignorance of chemistry. As Grisdale (2010) documents, even a slight change in the chemical constitution of water produces dramatic changes in its effects. If normal hydrogen is replaced by different isotopes, deuterium or tritium, the water molecule markedly changes its chemical properties. Life would be impossible if H2O were replaced by heavy water, D2O or T2O; and compounds made of elements different from hydrogen and oxygen would be even more different in their properties. Hence Putnam’s thought experiment is scientifically incoherent: If water were not H2O, Twin Earth would not be at all like Earth. [See also Universal Fire. --Luke]
This incoherence should serve as a warning to philosophers who try to base theories on thought experiments, a practice I have criticized in relation to concepts of mind (Thagard, 2010a, ch. 2). Some philosophers have thought that the nonmaterial nature of consciousness is shown by their ability to imagine beings (zombies) who are physically just like people but who lack consciousness. It is entirely likely, however, that once the brain mechanisms that produce consciousness are better understood, it will become clear that zombies are as fanciful as Putnam’s XYZ. Just as imagining that water is XYZ is a sign only of ignorance of chemistry, imagining that consciousness is nonbiological may well turn out to reveal ignorance rather than some profound conceptual truth about the nature of mind. Of course, the hypothesis that consciousness is a brain process is not part of most people’s everyday concept of consciousness, but psychological concepts can progress just like ones in physics and chemistry. [See also the Zombies Sequence. --Luke]
I think this radically misunderstands what thought experiments are for. As I see it, the job of philosophy is to clear up our own conceptual confusions; that's not the sort of thing that ever could conflict with science!
(EDIT: I mean that it shouldn't conflict with science; if you do your philosophy wrong then you might end up conflicting.)
Besides, Putnam's thought experiment can be easily tweaked to get around that problem: suppose that on Twin Earth cats are in fact very sophisticated cat-imitating robots. Then a similar conclusion follows about the meaning of "cat". The point is that if X had in fact been Y, where Y is the same as X in all the respects which we use to pick out X, then words which currently refer to X would refer to Y in that situation. I think Putnam even specifies that we are to imagine that XYZ behaves chemically the same as H2O. Sure, that couldn't happen in our world; but the laws of physics might have turned out differently, and we ought to be able to conceptually deal with possibilities like this.
I think this is wrong, and one of the major mistakes of 20th century analytic philosophy.
Just to be clear, I think that analytic philosophers often should have been more humble when they barged in and started telling scientist how confused they were. Fodor's critique of NS would again be my go-to example of that.
Dennett states this point in typically strong terms in his review of Fodor's argument:
It certainly can, if the job is done badly.
Agreed that Grisdale's argument isn't very good, I have a hard time taking Putnam's argument seriously, or even the whole context in which he presented his thought experiment. Like a lot of philosophy, it reminds me of a bunch of maths noobs arguing long and futilely in a not-even-wrong manner over whether 0.999...=1.
We on Earth use "water" to refer to a certain substance; those on Twin Earth use "water" to refer to a different substance with many of the same properties; our scientists and theirs meet with samples of the respective substances, discover their constitutions are actually diffferent, and henceforth change their terminology to make it clear, when it needs to be, which of the two substances is being referred to in any particular case.
There is no problem here to solve.
Well, sure, you can do philosophy wrong!
It sounds to me that you're expecting something from Putnam's argument that he isn't trying to give you. He's trying to clarify what's going on when we talk about words having "meaning". His conclusion is that the "meaning", insofar as it involves "referring" to something, depends on stuff outside the mind of the speaker. That may seem obvious in retrospect, but it's pretty tempting to think otherwise: as competent users of a language, we tend to feel like we know all there is to know about the meanings of our own words! That's the sort of position that Putnam is attacking: a position about that mysterious word "meaning".
EDIT: to clarify, I'm not necessarily in total agreement with Putnam, I just don't think that this is the way to refute him!
I've always thought this argument of Putnam's was dead wrong. It is about the most blatant and explicit instance of the Mind Projection Fallacy I know.
The real problem for Putnam is not his theory of chemistry; it is his theory of language. Like so many before and after him, Putnam thinks of meaning as being a kind of correspondence between words and either things or concepts; and in this paper he tries to show that the correspondence is to things rather than concepts. The error is in the assumption that words (and languages) have a sufficiently abstract existence to participate in such correspondences in the first place. (We can of course draw any correspondence we like, but it need not represent any objective fact about the territory.)
This is insufficiently reductionist. Language is nothing more than the human superpower of vibratory telepathy. If you say the word "chair", this physical action of yours causes a certain pattern of neurons to be stimulated in my brain, which bears a similarity relationship to a pattern of neurons in your brain. For philosophical purposes, there is no fact of the matter about whether the pattern of neurons being stimulated in my brain is &... (read more)
Putnam perhaps chose poor examples, but his thought-experiment works under any situation where we have limited knowledge.
Instead of Twin Earth, say that I have a jar of clear liquid on my desk. Working off of just that information (and the information that much of the clear liquid that humans keep around are water) people start calling the thing on my desk a "Jar of Water." That is, until someone knocks it over and it starts to eat through the material on my desk: obviously, that wasn't water.
Putnam doesn't think that XYZ will look like water in ... (read more)
I'm not sure that showing that XYZ can't make something water-like is any more helpful than just pointing out that there isn't actually a Twin Earth. Yes, it was supposed to be a counterfactual thought experiment. Oh noes, the counterfactual doesn't actually obtain!
And showing that particular chemical compounds don't make water, doesn't entail that there is no XYZ that makes water.
And as army1987 pointed out, it could have been "cat" instead of "water".
I'm going to agree with those saying that Thagard is missing the point of Putnam's thought experiment. Below, I will pick on Thagard's claim that Grisdale has refuted Putnam's thought experiment. For anyone interested, Putnam's 1975 article "The Meaning of "Meaning"", and Grisdale's thesis are both available as PDFs.
Thagard says that Grisdale has refuted Putnam's thought experiment. What would it mean to refute a thought experiment? I would have guessed that Thagard meant the conclusion or lesson drawn from the thought experiment is... (read more)
What a silly thought experiment. The fact that two people use one word to refer to two different things (which superficially appear similar) doesn't mean anything except that the language is imperfect.
Case in point: Uses of the word "love".
Pointing out that biochemistry couldn't be the same if water was different sounds like deliberately missing the point of Putnam's experiment. Suppose a planet like Earth, but where most people are left-handed, have their heart in the right-hand side of their body, wear wedding rings on their right hand, most live in the hemisphere where shadows move counterclockwise, most screws are left-handed, conservative political parties traditionally sit in the left-hand side of assemblies, etc., etc., and they speak a language identical to English except that left m... (read more)
It depends on your thought experiment - mathematics can be categorised as a form of thought experimentation, and it's generally helpful.
Thought experiments show you the consequences of your starting axioms. If your axioms are vague, or slightly wrong in some way, you can end up with completely ridiculous conclusions. If you are in a position to recognise that the result is ridiculous, this can help. It can help you to understand what your ideas mean.
On the other hand, it sometimes still isn't that helpful. For example, one might argue that an object can't ... (read more)
I think many of the other commenters have done an admirable job defending Putnam's usage of thought experiments, so I don't feel a need to address that.
However, there also seems to be some confusion about Putnam's conclusion that "meaning ain't in the head." It seems to me that this confusion can be resolved by disambiguating the meaning of 'meaning'. 'Meaning' can refer to either the extension (i.e. referent) of a concept or its intension (a function from the context and circumstance of a concept's usage to its extension). The extension clearly ... (read more)
Twin Earth is impossible in this universe. A universe could exist just like ours, except that water is made of a compound of xenon, yttrium, and zing (XeYZn). Furthermore, the laws of physics are such that this chemical acts like water does in ours, and everything else acts just like water in ours. The laws would have to be pretty bizarre, but they could exist.
This quote misunderstands the zombie thought experiment as used by Chalmers. Chalmers actually thinks zombies are impossible given the laws that actually govern the universe, and possible only in the sense it's possible the universe could have had different laws (or so many people would claim.)
I'm not as sure about Putnam's views, but I suspect he would make an analogous claim, that his thought experiment only requires Twin Earth to be possible in a very broad sense of possibility.
Putnam's flaw is to try to prescribe how language works. Putnam is like, language works like X because it has to, ignoring that we create language and can choose how it works. I'd agree with the suggestion further up that the typical mind fallacy is at work here.
A similar point is that a lot of bad theories historically are the result of trying to explain something that should just be taken as an irreducible primary. Aristotle tried to explain "motion" by means of the "unmoved mover", Newton was treated skeptically because his theory didn't explain why things continued to move, Lavoisier's theory of oxygen I think was treated similarly contra phlogiston.
I think something is missing here. Suppose that water has some unknown property Y that may allow us to do Z. This very statement requires that water somehow refers to object in the real world, so that we would be interested in experimenting with the water in the real world instead of doing some introspection into our internal notion of 'water'. We want our internal model of water to match something that is only fully defined externally.
Other example, if water is the only liquid we know, we may have combined notions of 'liquid' and 'water', but as we explo... (read more)
I don't think there is anything special about consciousness. "Consciousness" is what any intelligence feels from the inside, just as qualia are what sense perceptions feel like from the inside.