At their core externalist theories are based upon the principle that mental content can’t be strictly within the brain, or, in other words, that it can’t be defined in terms of psychological/mental properties that supervene on physical (non-relational) properties of the individual. Generally we understand mental content to be the meaning of words. It is meaning that distinguishes one idea from another, not the extension (the set of all objects in the world that the word “picks out”), since different ideas can have the same extension. By understanding mental content as meaning, which is how Putnam, the externalist whose arguments I will examine below, understands it, we can see how a specific theory of meaning, the theory that a word’s meaning is really some transformation rules from the symbol to a conscious act with certain satisfaction conditions (a certain horizon), an internal definition, refutes or is refuted by the arguments for externalism.
Putnam’s classic argument for externalism, presented in “The Meaning of ‘Meaning’”, is the Thin Earth thought experiment. In it Putnam asks us to consider two almost identical worlds, Earth and Twin Earth, which differ only in the fact that water on Earth is H2O and water on Twin Earth is XYZ . Since they are otherwise the same the people on Earth and Twin Earth will have the exact same psychological properties, which means that if meaning is internal “water” will mean the same thing to individuals in both worlds. However, water on Twin Earth is not the same as water on Earth, and so they really can’t mean the same thing. Thus meaning must not be something that can be defined purely internally.
If we accepted the reasoning presented above this would indeed be a paradox for internalism about meaning, since if two words have the same meaning they must pick out the same extension, the same stuff in the world, and it seems like in the case of the Twin Earth thought experiment they don’t. It only seems like this is the case though, what is really happening is that the reader is being mislead into confusing the reference with the extension. Even if two words mean the same thing they may have a different reference, and the same word may have a different reference when used in different circumstances, because the reference is the thing or things that are causally connected with the speaker which have brought about his or her use of the word; it is the stuff they are talking about. On Earth the reference of water is H2O because that is the physical stuff that people are talking about when they talk about water, it is the stuff that they have learned to associate the word water with. Similarly on Twin Earth the reference of water is XYZ, for the same reasons. The extension, however, is not defined strictly as whatever the people who use the word are talking about, the extension is everything that meets the “satisfaction conditions” for that word (for water it is being wet, drinkable, ect). For people on Earth and Twin Earth their satisfaction conditions for “water” are the same, since satisfaction conditions are determined by the transformation rules for the word, which are psychological properties, and thus the same for both worlds, by hypothesis. And since their satisfaction conditions are the same both H2O and XYZ are in the extension of water, and since they have the same extension there is no reason to believe that water means anything different for people of Earth and Twin Earth.
There are two possible ways for externalists to respond to this. One is to say that later people living on Earth and Twin Earth might come to discover that water is made of H2O and XYZ respectively (since the worlds were exactly alike we were assuming they hadn’t discovered chemistry yet), and thus by our earlier definition of meaning come to mean different things by water (since different things would now meet the satisfaction conditions), but clearly the water in the world that they were talking about hasn’t changed, which is a contradiction. Again, this response confuses a fixed reference for the word’s meaning, so given the explanation above I don’t think I need to go into any more detail. A more interesting case (not presented in Putnam’s original article) is to consider Twin Earth and Earth again, but this time the people on Twin Earth have a different physiology from us, they are quenched by acids and burned by water, and in their world clear acid is found wherever water is in ours . In both worlds the inhabitants could have the same transformation rules for “water”, and thus the same meaning (for example, water is something that quenches thirst), but the word certainly has a different extension for people on Earth and this new Twin Earth. This response goes wrong in assuming that the transformation rules for people on Earth and this Twin Earth really are the same. Specifically the difference lies in subject relative transformation rules, for example: water is quenching [to people like me]. I have added “to people like me” in brackets because it is a hidden assumption that is part of all “subjective” transformation rules. Clearly “people like me” is one set of individuals for people on Earth and another for people on Twin Earth, since they don’t have the same physiologies (unlike the original thought experiment, which is why we could not give this response to the earlier version), and so their transformation rules really aren’t the same, and hence water doesn’t really mean the same thing to inhabitants of both worlds .
Obviously this isn’t a disproof of externalism. It does, however, show that there are ways to define meaning in a completely internal fashion that aren’t contradictory. Some may define meaning in terms of the extension or reference of words from the outset, and for them meaning, and mental content, is by necessity external. However, such a definition of meaning can’t be an argument in favor of externalism, since it begs the question. Of course if I find any arguments against the possibility of meaning being defined internally that don’t rest on a confusion between reference and extension, or what it takes to mean the same thing, I will do my best to address them, so feel free to submit links to papers.
1. There is actually some confusion about how this could be even possible, since if the worlds are to be exactly the same in every other ways besides this fact then it would seem that XYZ and H2O have the same causal powers, which would make them impossible to distinguish, at which point what reason do we have to say that they aren’t the same thing? However, let us just assume for the moment that such a pair of worlds is possible.
2. Again, we will have to ignore the physical impossibility of this.
3. Some may take “people like me” to be working in a hidden kind of externalism. If this bothers you then we could simply add [to me] in the brackets, and then since we recover objective meaning (determining when two people mean the same thing by their transformation rules) by requiring that the rules be similar we can require that the “me”s be sufficiently similar as well. And this definitely isn’t a hidden kind of externalism, since we already had to define objective or shared meaning by similarities, adding one more comparison doesn’t make a difference.