Putnam offers the now famous Twin Earth thought experiment in order to defend the thesis that the meaning of a term is at least in part settled by what exists in the world.
HILARY PUTNAM
Meaning and Reference
Unclear as it is, the traditional doctrine that the notion âmeaningâ possesses the extension/intension ambiguity has certain typical consequences. The doctrine that the meaning of a term is a concept carried the implication that meanings are mental entities. Frege, however, rebelled against this âpsychologism.â Feeling that meanings are public propertyâthat the same meaning can be âgraspedâ by more than one person and by persons at different timesâhe identified concepts (and hence âintensionsâ or meanings) with abstract entities rather than mental entities. However, âgraspingâ these abstract entities was still an individual psychological act. None of these philosophers doubted that understanding a word (knowing its intension) was just a matter of being in a certain psychological state (somewhat in the way in which knowing how to factor numbers in oneâs head is just a matter of being in a certain very complex psychological state).
Secondly, the timeworn example of the two terms âcreature with a kidneyâ and âcreature with a heartâ does show that two terms can have the same extension and yet differ in intension. But it was taken to be obvious that the reverse is impossible: two terms cannot differ in extension and have the same intension. Interestingly, no argument for this impossibility was ever offered. Probably it reflects the tradition of the ancient and medieval philosophers, who assumed that the concept corresponding to a term was just a conjunction of predicates, and hence that the concept corresponding to a term must always provide a necessary and sufficient condition for falling into the extension of the term. For philosophers like Carnap, who accepted the verifiability theory of meaning, the concept corresponding to a term provided (in the ideal case, where the term had âcomplete meaningâ) a criterion for belonging to the extension (not just in the sense of ânecessary and sufficient condition,â but in the strong sense of way of recognizing whether a given thing falls into the extension or not). So theory of meaning came to rest on two unchallenged assumptions:
I shall argue that these two assumptions are not jointly satisfied by any notion, let alone any notion of meaning. The traditional concept of meaning is a concept which rests on a false theory.
Are Meanings in the Head?
For the purpose of the following science-fiction examples, we shall suppose that somewhere there is a planet we shall call Twin Earth. Twin Earth is very much like Earth: in fact, people on Twin Earth even speak English. In fact, apart from the differences we shall specify in our science-fiction examples, the reader may suppose that Twin Earth is exactly like Earth. He may even suppose that he has a Doppelgangerâan identical copyâon Twin Earth, if he wishes, although my stories will not depend on this.
Although some of the people on Twin Earth (say, those who call themselves âAmericansâ and those who call themselves âCanadiansâ and those who call themselves âEnglishmen,â etc.) speak English, there are, not surprisingly, a few tiny differences between the dialects of English spoken on Twin Earth and standard English.
One of the peculiarities of Twin Earth is that the liquid called âwaterâ is not H2O but a different liquid whose chemical formula is very long and complicated. I shall abbreviate this chemical formula simply as XYZ. I shall suppose that XYZ is indistinguishable from water at normal temperatures and pressures. Also, I shall suppose that the oceans and lakes and seas of Twin Earth contain XYZ and not water, that it rains XYZ on Twin Earth and not water, etc.
If a space ship from Earth ever visits Twin Earth, then the supposition at first will be that âwaterâ has the same meaning on Earth and on Twin Earth. This supposition will be corrected when it is discovered that âwaterâ on Twin Earth is XYZ, and the Earthian space ship will report somewhat as follows.
âOn Twin Earth the word âwaterâ means XYZ.â
Symmetrically, if a space ship from Twin Earth ever visits Earth, then the supposition at first will be that the word âwaterâ has the same meaning on Twin Earth and on Earth. This supposition will be corrected when it is discovered that âwaterâ on Earth is H2O, and the Twin Earthian space ship will report:
âOn Earth the word âwaterâ means H2O.â
Note that there is no problem about the extension of the term âwaterâ: the word simply has two different meanings (as we say); in the sense in which it is used on Twin Earth, the sense of waterTE, what we call âwaterâ simply isnât water, while in the sense in which it is used on Earth, the sense of waterE, what the Twin Earthians call âwaterâ simple isnât water. The extension of âwaterâ in the sense of waterE is the set of all wholes consisting of H2O molecules, or something like that; the extension of water in the sense of waterTE is the set of all wholes consisting of XYZ molecules, or something like that.
Now let us roll the time back to about 1750. The typical Earthian speaker of English did not know that water consisted of hydrogen and oxygen, and the typical Twin-Earthian speaker of English did not know that âwaterâ consisted of XYZ. Let Oscar1 be such a typical Earthian English speaker, and let Oscar2 be his counterpart on Twin Earth. You may suppose that there is no belief that Oscar1 had about water that Oscar2 did not have about âwater.â If you like, you may even suppose that Oscar1 and Oscar2 were exact duplicates in appearance, feelings, thoughts, interior monologue, etc. Yet the extension of the term âwaterâ was just as much H2O on Earth in 1750 as in 1950; and the extension of the term âwaterâ was just as much XYZ on Twin Earth in 1750 as in 1950. Oscar1 and Oscar2 understood the term âwaterâ differently in 1750 although they were in the same psychological state, and although, given the state of science at the time, it would have taken their scientific communities about fifty years to discover that they understood the term âwaterâ differently. Thus the extension of the term âwaterâ (and, in fact, its âmeaningâ in the intuitive preanalytical usage of that term) is not a function of the psychological state of the speaker by itself.1
But, it might be objected, why should we accept it that the term âwaterâ had the same extension in 1750 and in 1950 (on both Earths)? Suppose I point to a glass of water and say âthis liquid is called water.â My âostensive definitionâ of water has the following empirical presupposition: that the body of liquid I am pointing to bears a certain sameness relation (say, x is the same liquid as y, or x is the same Las y) to most of the stuff I and other speakers in my linguistic community have on other occasions called âwater.â If this presupposition is false because, say, I amâunknown to meâpointing to a glass of gin and not a glass of water, then I do not intend my ostensive definition to be accepted. Thus the ostensive definition conveys what might be called a âdefeasibleâ necessary and sufficient condition: the necessary and sufficient condition for being water is bearing the relation sameL to the stuff in the glass; but this is the necessary and sufficient condition only if the empirical presupposition is satisfied. If it is not satisfied, then one of a series of, so to speak, âfallbackâ conditions becomes activated.
The key point is that the relation sameL is a theoretical relation: whether something is or is not the same liquid as this may take an indeterminate amount of scientific investigation to determine. Thus, the fact that an English speaker in 1750 might have called XYZ âwater,â whereas he or his successors would not have called XYZ water in 1800 or 1850 does not mean that the âmeaningâ of âwaterâ changed for the average speaker in the interval. In 1750 or in 1850 or in 1950 one might have pointed to, say, the liquid in Lake Michigan as an example of âwater.â What changed was that in 1750 we would have mistakenly thought that XYZ bore the relation sameL to the liquid in Lake Michigan, whereas in 1800 or 1850 we would have known that it did not.
Let us now modify our science-fiction story. I shall suppose that molybdenum pots and pans canât be distinguished from aluminum pots and pans save by an expert. (This could be true for all I know, and, a fortiori, it could be true for all I know by virtue of âknowing the meaningâ of the words aluminum and molybdenum.) We will now suppose that molybdenum is as common on Twin Earth as aluminum is on Earth, and that aluminum is as rare on Twin Earth as molybdenum is on Earth. In particular, we shall assume that âaluminumâ pots and pans are made of molybdenum on Twin Earth. Finally, we shall assume that the words âaluminumâ and âmolybdenumâ are switched on Twin Earth: âaluminumâ is the name of molybdenum, and âmolybdenumâ is the name of aluminum. If a space ship from Earth visited Twin Earth, the visitors from Earth probably would not suspect that the âaluminumâ pots and pans on Twin Earth were not made of aluminum, especially when the Twin Earthians said they were. But there is one important difference between the two cases. An Earthian metallurgist could tell very easily that âaluminumâ was molybdenum, and a Twin Earthian metallurgist could tell equally easily that aluminum was âmolybdenum.â (The shudder quotes in the preceding sentence indicate Twin Earthian usages.) Whereas in 1750 no one on either Earth or Twin Earth could have distinguished water from âwater,â the confusion of aluminum with âaluminumâ involves only a part of the linguistic communities involved.
This example makes the same point as the preceding example. If Oscar1 and Oscar2 are standard speakers of Earthian English and Twin Earthian English, respectively, and neither is chemically or metallurgically sophisticated, then there may be no difference at all in their psychological states when they use the word âaluminumâ; nevertheless, we have to say that âaluminumâ has the extension aluminum in the idiolect of Oscar1 and the extension molybdenum in the idiolect of Oscar2· (Also we have to say that Oscar1 and Oscar2 mean different things by âaluminumâ; that âaluminumâ has a different meaning on Earth than it does on Twin Earth, etc.) Again we see that the psychological state of the speaker does not determine the extension (or the âmeaning,â speaking preanalytically) of the word.
Before discussing this example further, let me introduce a non-science-fiction example. Suppose you are like me and cannot tell an elm from a beech tree. We still say that the extension of âelmâ in my idiolect is the same as the extension of âelmâ in anyone elseâs, viz., the set of all elm trees, and that the set of all beech trees is the extension of âbeechâ in both of our idiolects. Thus âelmâ in my idiolect has a different extension from âbeechâ in your idiolect (as it should). Is it really credible that this difference in extension is brought about by some difference in our concepts? My concept of an elm tree is exactly the same as my concept of a beech tree (I blush to confess). If someone heroically attempts to maintain that the difference between the extension of âelmâ and the extension of âbeechâ in my idiolect is explained by a difference in my psychological state, then we can always refute him by constructing a âTwin Earthâ exampleâjust let the words âelmâ and âbeechâ be switched on Twin Earth (the way âaluminumâ and âmolybdenumâ were in the previous example). Moreover, suppose I have a doppelganger on Twin Earth who is molecule for molecule âidenticalâ with me. If you are a dualist, then also suppose my Doppelganger thinks the same verbalized thoughts I do, has the same sense data, the same dispositions, etc. It is absurd to think his psychological state is one bit different from mine: yet he âmeansâ beech when he says âelm,â and I âmeanâ elm when I say âelm.â Cut the pie any way you like, âmeaningsâ just ainât in the head!
A Sociolinguistic Hypothesis
The last two examples depend upon a fact about language that seems, surprisingly, never to have been pointed out: that there is division of linguistic labor. We could hardly use such words as âelmâ and âaluminumâ if no one possessed a way of recognizing elm trees and aluminum metal; but not everyone to whom the distinction is important has to be able to make the distinction. Let us shift the example; consider gold. Gold is important for many reasons: it is a precious metal; it is a monetary metal; it has symbolic value (it is important to mo...