After some introductory remarks, Putnam begins by sketching and then criticizing the traditional distinction between intension and extension. As he notes, intensions were often taken to be "concepts" and therefore to be a sort of mental entity. Frege—think here of his remarks about ideas—rejected this view, insisting that senses are objective. But, as Putnam notes, Frege nonetheless regards grasping a sense as a psychological act or state. Frege of course also held that sense determines reference, or that intension determines extension.
Thus, Putnam claims, traditional views about meaning are committed to two claims:
- "...[K]nowing the meaning of a term is just a matter of being in a certain pyschological state..." (p. 135).
- "...[T]he [intension] of a term...determines its extension..." (p. 136).
Putnam's central claim is that these two conditions are incompatible, and that is because they jointly entail:
- 3. The extension that a term has for a given speaker is determined by that speaker's psychological state.
The argument for (3) is that (1) entails that what intension a term has for the speaker is determined by her psychological state; the intension then determines the extension, by (2). (See pp. 137-8.)
One of the most important issues here is exactly what a "psychological state" is supposed to be, and Putnam has in mind a very particular conception of such a state. He explains on pp. 136-9 that he takes "traditional" philosophy to be committed to a doctrine he calls methodological solipsism but is nowadays often called "Cartesianism": whether someone is in a given psychological state depends upon nothing "outside" that particular individual; it does not depend upon the existence of any other individual, nor even of the external world.
To be kind, it isn't entirely clear what Putnam means by a psychological state in the narrow sense. In fact, Putnam expresses a great deal of skepticism about the very notion. (See the remarks on jealousy, etc.) But one way to try to make it more precise would be to say that, at the very least, if two people have all their intrinsic physical properties in common, then they must have all their (narrow) psychological properties in common. Explain why that helps. Or not. (Hint: This would assume a weak form of mental–physical supervenience.)
We can understand this as a sort of supervenience claim: What psychological states one is in depend only upon (i.e., supervene upon) one's "intrinsic" features, i.e., upon how things are "from the skin in". Putnam calls pyschological states that that satisfy this sort of condition "pyschological states in the narrow sense" and then says that he will understand (1) above as:
- 1'. Knowing the meaning of a term is a matter of being in a certain pyschological state in the narrow sense.
We can therefore also strengthen (3) to:
- 3'. The extension that a term has for a given speaker is determined by that speaker's psychological state in the narrow sense.
- 4. Hence, if two speakers are in the same psychological state in the narrow sense, then each term they understand must have the same extension for them.
It is (4) that is the target of the examples that follow.
On pp. 138-9, Putnam argues that Frege's insistence that senses are public and objective is really irrelevant to the central issues. Why? And does that seem right?
What follows, on pp. 139-42, is the now famous "Twin Earth" thought experiment. Putnam has us imagine a distant planet that is exactly like Earth except that what's in the lakes, etc, isn't H2O but something else: XYZ. The argument then proceeds in a few steps:
- If astronauts today were to visit Twin Earth, they'd report back that there was no water in the lakes there, but some other substance, XYZ, and that the Twin Earth term "water" didn't mean water at all, but 'twater' (which is XYZ).
- Even before the advent of chemistry, it was still true that, on Earth, "water" was true only of H2O and, by parity, on Twin Earth, only of XYZ.
- Now let Twin Oscar be Oscar's physical duplicate on Twin Earth in 1750. Then Oscar and his Twin are in the same psychological state in the narrow sense, but the term "water" has different extensions for them.
As Putnam notes, it's crucial to this particular thought experiment that "water" is a "natural kind term": It's a presupposition of our use of it that we use it to pick out a certain kind of stuff; water is whatever is the same kind of stuff as typical examples of water. And what "same kind of stuff" means will not necessarily be obvious but require empirical investigation.
It is often pointed out that, since people are mostly water, there is no way that Twin Oscar can be a physical duplicate of Oscar. How serious is this worry?
The conclusion we are meant to draw from this example is that the extension that a word has for a particular speaker can depend upon facts about the physical environment in which she lives. More precisely, much as with Kripke, the thought is that what extension a term has for someone is in part a matter of causal relationships between her and her physical environment. This view is nowadays known as (semantic) externalism, in contrast to internalism, which is the view Putnam is arguing against.
On p. 143, Putnam introduces another now-famous example, which he takes to be of roughly the same form. Putnam says that he cannot personally tell elms from beeches (though let's assume he can tell them from other sorts of trees), then claims that his "concept" of a beech is the same as his "concept" of an elm. (I take it that he means he has the same beliefs about each, so that the "cluster of descriptions" he associates with each is the same.) Nonetheless, Putnam claims, his word "elm" applies only to elms and his word "beech" applies only to beeches: If he points at a beech and says "That's an elm", he speaks falsely. But there is a different Twin Earth on which the words "elm" and "beech" are switched, so that Twin Putnam's word "elm" applies to beeches.
Putnam takes this example to show "that there is a division of linguistic labor" (p. 144): People who do not know how to identify elms and beeches (or whatever) can depend upon people who do, and the extension the word has for me can be determined by the extension it has for the "experts". Thus, as in the original Twin Earth case, the lesson is supposed to be that a person's "individual psychological state...does not fix [the] extension" of a term that is subject to a division of linguistic labor (p. 146).
So the conclusion we are meant to draw from this sort of example is that the extension a word has for a particular speaker can depend upon the social environment in which she lives. This view is nowadays known as anti-individualism, in contrast to individualism, which is the view Putnam is arguing against.
Like the H2O–XYZ example, the one Putnam uses to argue for anti-indvidualism also involves "natural kind" terms: "elm" and "beech", which are of course names of types of trees. Can you come up with a similar example that doesn't involve natural kind terms? Can you come up with an example that might be used to argue for externalism that doesn't involve natural kind terms?
Suppose Putnam is correct that (1) and (2) above are incompatible. Which should be jettisoned? Should we conclude that knowing the meaning of a term is not just a matter of being in a certain psychological state in the narrow sense? I.e., that Oscar and Twin Oscar disagree about what "water" means? Or should we conclude that intension does not determine extension? I.e., that "water" means the same for them, but applies to different things? For that matter, is either option comfortable?