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.)

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    Good 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
    Commented Apr 9, 2012 at 16:50

3 Answers 3


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.

  • 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
    Commented Apr 11, 2012 at 14:42
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    @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. Commented Apr 11, 2012 at 22:35
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    +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". Commented Apr 12, 2012 at 8:31
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    @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
    Commented Apr 13, 2012 at 7:28
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    @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. Commented Apr 13, 2012 at 11:55

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?)

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    Maybe 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
    Commented Apr 10, 2012 at 13:03
  • @adrianos - Good point--but you've already made it in the comment, so I'll leave it there!
    – Rex Kerr
    Commented Apr 10, 2012 at 14:41

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.

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