How does the Twin Earth illustration show that "meanings just aren't in the head?"


Hiliary Putnam's Twin Earth thought experiment goes like this:

We begin by supposing that elsewhere in the universe there is a planet exactly like earth in virtually all respects, which we refer to as ‘Twin Earth’. (We should also suppose that the relevant surroundings [are exactly the same as for] earth; it revolves around a star that appears to be exactly like our sun, and so on.) On Twin Earth there is a Twin equivalent of every person and thing here on Earth. The one difference between the two planets is that there is no water on Twin Earth. In its place there is a liquid that is superficially identical, but is chemically different, being composed not of H2O, but rather of some more complicated formula which we abbreviate as ‘XYZ’. The Twin Earthlings who refer to their language as ‘English’ call XYZ ‘water’. Finally, we set the date of our thought experiment to be several centuries ago, when the residents of Earth and Twin Earth would have no means of knowing that the liquids they called ‘water’ were H2O and XYZ respectively. The experience of people on Earth with water, and that of those on Twin Earth with XYZ would be identical.

Now the question arises: when an earthling (or Oscar for simplicity sake) and his twin on Twin Earth say 'water' do they mean the same thing? (The twin is also called 'Oscar' on his own planet, of course. Indeed, the inhabitants of that planet call their own planet 'Earth'. For convenience, we refer to this putative planet as 'Twin Earth', and extend this naming convention to the objects and people that inhabit it, in this case referring to Oscar's twin as Twin-Oscar, and twin-earth water as water.) Ex hypothesi, their brains are molecule-for-molecule identical. Yet, at least according to Putnam, when Oscar says water, the term refers to H2O, whereas when Twin Oscar says 'water' it refers to XYZ. The result of this is that the contents of a person's brain are not sufficient to determine the reference of terms they use, as one must also examine the causal history that led to this individual acquiring the term. (Oscar, for instance, learned the word 'water' in a world filled with H2O, whereas Twin Oscar learned 'water' in a world filled with XYZ.)

Kevin Davis

Posted 2012-04-09T16:08:41.623

Reputation: 439

2Good question, but I think it would be helpful if you provided a little elaboration on the phrase "meanings just aren't in the head." As it is, the question seems a tad ambiguous and I think it could be made more clear if you explained your concern a little more. – commando – 2012-04-09T16:50:58.153



The Twin Earth thought experiment does not show that "meanings just aren't in the head”.

It is a flawed "Intuition pump", and Putnam's conclusion was flawed.

Consider that Oscar and Twin-Oscar, and their worlds, are identical in every way (but the one). The properties of H2O and XYZ are exactly the same; they have to be or the worlds could not otherwise be identical.

Oscar's "water" = H2O and Twin-Oscar's "water" = XYZ. If you teleported Oscar to Twin-Earth and zapped Twin-Oscar to Earth, Oscar would see that XYZ was also "water" and Twin-Oscar would see that H2O was "water".

Since by all their observable traits, both compounds behaved the same way, both Oscar and Twin-Oscar would agree that both H2O and XYZ were "water".

Both would be accurate. Among the mistakes Putnam makes is confusing accuracy for precision.

Analyzing the state of Oscar's mind would allow one to determine, detect, or define "water" just as well as either Oscar or Twin Oscar, on either world. The information may not be precise enough to tell the XYZ variant of "water" from the H2O variant of "water", but that does not mean that the definition is not accurate and the information "in the head" is not sufficient.

Brock Adams

Posted 2012-04-09T16:08:41.623

Reputation: 294

BTW, since the brain contains H2O-water, the twin-brain necessarily would contain XYZ-water instead (since there's no H2O on the twin planet), and therefore would not be completely identical to the original brain anyway. – celtschk – 2014-08-27T23:00:39.150

Since this is a thought-experiment, drinking XYZ might cause Oscar to explode in a huge and violent reaction. This is not what water does, so Oscar would be correct to say that it was not his water. – Rex Kerr – 2012-04-11T14:42:59.463

2@RexKerr: No, their worlds are identical except in that one respect (a physical impossibility, BTW), and their brains are "molecule-for-molecule identical" (which means that their bodies must be as well). Even in a thought experiment, there is no substance that can react so differently for Oscar than for twin Oscar. – Brock Adams – 2012-04-11T22:35:34.147

1+1 Thanks for trying to explain why Putnam's argument is flawed. I thought about how to explain that his assumption of "Twin Earth" is simply too "flawed"/"physically impossible" to illustrate anything. But because any though experiment will be slightly flawed, it's difficult to draw the line. But his thought experiment is not even "relevant enough" to what he actually want to illustrate, so it is significantly more flawed than for example John Searle's "Chinese room". – Thomas Klimpel – 2012-04-12T08:31:32.937

@BrockAdams - Twin Oscar might be made out of antimatter and XYZ might be anti-H2O. – Rex Kerr – 2012-04-12T14:46:50.467

@RexKerr, only at first approximation, the worlds could not then be identical, and Twin Oscar's world would be a hellish nightmare as the rest of the universe worked to annihilate it. There is a reason that there is almost no antimatter left in the universe. H2O = XYZ is a physical impossibility, even if XYZ is antimatter or heavy water (chemically identical to H2o, but not physically). Next you will say "alternate universe" (which Putnam did not). Then the premise could work, but both men would still rightly ID both substances as "water", and maybe realize that they needed more precision. – Brock Adams – 2012-04-12T22:52:01.373

@BrockAdams - I think you're missing the point of the thought experiment. The physics is questionable (even if there is something which is perhaps surprisingly close), and that is what you're quibbling with. The logic is independent of that exact physics. It's not that the worlds are identical in every way, it's that they're identical in every way that Oscar and Twin-Oscar can detect themselves. – Rex Kerr – 2012-04-13T06:46:03.640

@RexKerr, I showed that the logic was wrong. "Quibbling" with the physics only came later, as an aside, in the *comments*. Now you are trying to retroactively change the whole premise. What part of Putnam's "The one difference between the two planets is that there is no water..." is not clear? – Brock Adams – 2012-04-13T06:54:56.753

@BrockAdams - "Finally, we set the date of our thought experiment to be several centuries ago, when the residents of Earth and Twin Earth would have no means of knowing that the liquids they called ‘water’ were H2O and XYZ respectively." Thus, clearly, they are distinguishable physically, just not with technology available 200 years ago. You are necessarily quibbling with the physics if you dispute that this is possible; you are the one who asserted that their properties must be exactly the same. – Rex Kerr – 2012-04-13T06:58:35.840

E = (A + H) and TE = (A + X). Plus, TE = E, by all observable means. Therefor (A + H) = (A + x) which means that H = X -- by every criteria known to either observer, AND as required for the worlds to have only "one difference". It's fundamental logic. – Brock Adams – 2012-04-13T07:13:52.923

1@BrockAdams - By every criteria known to either observer at the time, yes. You are inappropriately discounting the point of the "Finally, we set the date" statement. – Rex Kerr – 2012-04-13T07:28:05.943

-1 They agree, they both use the term 'water', they both believe they mean the same thing by their words. But ex hypothesi H2O != XYZ, so they in fact mean different things by their words. So meanings cannot be determined by belief. If Putnam made any mistake, it was in thinking that the argument was limited to natural kinds when as Tyler Burge has argued, it applies to any ostensibly definable type. – adrianos – 2012-04-13T11:05:43.833

1@adrianos: The meaning of "water" correctly applies to both substances - whether or not some advanced instrument could further divide that water into 2 types -- using facts unknown to the men who assigned the meaning. By the insane logic of Putnam, a lime-colored table is not correctly called a "table" by a color blind man who is only used to avocado-colored tables -- in an era before color charts, no less. – Brock Adams – 2012-04-13T11:55:41.717

@BrockAdams Yes they surely both use the term 'water' correctly, but the 'facts unknown' to them are precisely what are not 'in the head', and these facts determine the what they mean. I don't see that your conclusion about the colourblind man follows at all. On Putnam's logic, such a person would obviously call the thing an avocado-coloured table, and that is what we should expect him to call it. Here the meaning may be 'in his head', but Putnam never denies this. His argument is applied only to natural kind terms. – adrianos – 2012-04-13T12:07:38.323


The conclusion is that Oscar's "water" is not Twin-Oscar's "water", but you can only know that by looking outside their heads, because they don't know it (at least not yet); yet you know that they mean different things because if Oscar knew that "H2O" and "XYZ" both existed and were not the same, he would (presumably) insist that only H2O was actually what he meant by "water". (He meant his water, not Twin-Oscar's twin-water.)

(Is that the step that you were finding problematic?)

Rex Kerr

Posted 2012-04-09T16:08:41.623

Reputation: 15 388

1Maybe you could add that what Putnam means by 'in the head' includes not only mental states, such as their beliefs about 'water', but also physical states, i.e. the actual brain state. Neither of these states are sufficient to determine the reference of 'water'. – adrianos – 2012-04-10T13:03:02.777

@adrianos - Good point--but you've already made it in the comment, so I'll leave it there! – Rex Kerr – 2012-04-10T14:41:06.037


I have been puzzled by this thought experiment for some time. It seems to me that the main flaw is that if the liquid on twin-earth is not H2O, and twin-Oscar is biologically identical to Oscar (and us), then if twin -Oscar drinks xyz he will die, since humans need H2O - unless the xyz is some kind of safe H2O based liquid.

ian bisset

Posted 2012-04-09T16:08:41.623

Reputation: 1