Twin Earth


The ability to imagine worlds parallel to ours might sound like the realm of science fiction novelettes and television space dramas. However, imagining an exact replica of Earth opens very interesting insights on the nature of meaning, language and even behavioral psychology. Hilary Putnam was one of the first philosophers to use the opportunities of a Twin Earth filled with exact replicas of every inhabitant on earth to challenge the traditional theories of meaning and reference in human language. Philosophers such as Kripke and Teller supported and followed Putnam into exploring the implications of her newly created framework while others like Zemach were opposed to her findings. Amongst the supporters, Stephen P. Stich uses the Twin Earth argument to point out the shortcomings of using belief-desire theories to explain causes of behavior in psychology. On this article I aim to explore Putnam’s twin earth and her challenge to traditional meaning and reference. On the second part I show how Stich makes use of this framework against belief-desire psychological theories.

In order to explore Putnam’s views on the nature of meaning we need to first refer to Frege’s theory. Also called the “traditional” theory, it puts forward a strong element of duality. Frege explains there are two components to meaning, namely sense or intension and reference or extension. These two are intimately related. The sense of a sentence translates into a proposition as a basic unit of communication with its extension being its truth value (i.e. true or false). Frege also coined a principle, namely that the semantic value of the whole is the combination of the semantic value of its parts and the way they are combined. So to understand something means understanding its parts. Furthermore, we can understand sentences with false extensions because we can know what the world would have to be like for the sentence to be true. With all this, Putnam puts together what she believes to be the two basic assumptions of traditional theory of meaning. First, knowing the meaning of a term is being in a certain psychological state. Second, the meaning of a term determines its extension. These two assumptions however, are never satisfied by any notion, according to Putnam. She states that the traditional idea of meaning rests on a false theory.

To show this, Putnam tries to give an argument to counter the standard belief that there cannot be two terms with the same extension but just one intension. She sets up the “Twin Earth” experiment in which we are asked to imagine there is a planet identical to Earth whose every inhabitant is a doppleganger of someone on earth. Everything on Twin Earth is exactly the same as on Earth, with the exception of water. While sharing the same evident properties with Earth water (including its name), the composition of substance from Twin Earth  is XYZ rather than H2O. Say we have two individuals, one on Earth and one on Twin Earth, and suppose they are not aware of the composition of what each calls “water”. Now, these two individuals understand the term “water” differently whilst being on the same psychological state, since the term water refers to two different objects. Therefore, she proves the extension of water is not dependent on the speakers internal psychological state by showing two terms with same intension and different extension. With this Putnam defies the idea of meaning as concepts inside the head of the speaker and puts forward the “Sociolinguistic Hypothesis”. She compares the linguistic community with a factory in which every individual has a different “job”. So if we use gold as an example, some people would have the “job” of wearing gold, others of standardising gold and only a few would study the actual properties and composition of gold. It is not necessary for everyone to know what the molecular structure of gold is, just to acquire the word “gold”. She calls this the division of linguistic labour. Thus, it is the sociolinguistic state of the collective linguistic body and not the speaker who fixes the extension of a given word.

Stich draws on Putnam’s work to show that beliefs are flawed as explanatory psychological devices in what he calls the “belief-desire” theory. He first defines beliefs to be of a “type-token” nature. This quite technical definition means that belief tokens can be seen as discrete states of a person. He sees states as the instantiation of a property by an object during a time interval. With this distinction, he goes on to draw a sufficient condition for non-identity of beliefs, namely that two beliefs which differ in truth value cannot be the same belief. In this line, he tries to show two belief states that are type identical while differing in truth value. If this can be proven, then these states cannot be beliefs as ordinarily understood. To show that, he works from within the principle of psychological autonomy and draws a series of examples (all very similar). Two of these take on Putnam’s “Twin Earth” thought exercise. Stich asks us to imagine that the philosopher Saul Kripke (born in Nebraska) was born in South Dakota on Twin Earth. However, Stich’s doppelgänger and most people in Twin Earth believe Kripke to be born in Nebraska. It can then be said that both Stich and his doppelgänger have the same belief about Kripke’s birthplace, but while Earth Stich’s belief is true, his paperhanger’s is false. Now, according to the autonomy principle, doppelgänger are supposed to share the same explanatory psychological properties. However, according to the identity principle, two beliefs that differ in truth value cannot be the equal. Therefore these beliefs cannot be explanatory properties of the individuals.

Still in Twin Earth, Stich asks us to imagine yet another example. Back in 1700, when the composition of water was unknown, Stich’s ancestor and his doppelgänger learn that there is a species of lizard that is soluble in water and they set out to dip the  However, this story is only true on Twin Earth, where water is XYZ. The lizard doesn’t dissolve in H2O. Any belief theorist would attribute the belief that lizards dissolve in water as one of the causes for the experiments. Again, the beliefs cannot be the same as they have different truth values. However, the individuals are physically identical so these beliefs cannot be part of an explanatory psychological theory. With this in hand, Stich is trying to show that beliefs are of what we could call a hybrid nature and thus a more complex analysis is required. Beliefs can be divided in two classes, namely de re and de dicto beliefs. While the distinction is far too complex for the scope of this essay, it is important to note that the previous Twin Earth examples are all de re beliefs, and these are in principle composed of de dicto beliefs. Only the latter however are compatible with the psychological autonomy principle. Stich then renames these belief states as autonomous. He states that the examples from Twin earth are all non-autonomous and points out that they could be analyzed in terms of autonomous beliefs. However, the state of exhibiting one or more autonomous properties shows no intrinsic truth value. Thus, if we strip off the non-autonomous properties of these beliefs then these will lack a truth value and by definition stop being beliefs as traditionally understood.

While Stich’s reasoning is sound, he bases its critique on the lack of arguments of both sides. Literally, he is neither able to find an argument to prove that non-autonomous beliefs can be analysed in terms of autonomous beliefs, nor can he find an argument to potentially disprove this; he just acknowledges that there might be a possible analysis. He does acknowledge the fact that his examples are somewhat flawed since the differences in truth value on the Twin Earth cases were due to disparities on reference. All in all, Stich’s argument, while flawed to some extent, fulfils his objective of starting a dialogue in what evidently is not an easy task. The same can be said of Putnam’s argument. These two philosophers challenge the traditional views of beliefs and meaning. As it is usually the case with most new views there are a certain number of metaphysical issues, such as the lack of counter arguments for or against Stich’s compositional view of beliefs. Putnam’s points are also subject to criticism: it is hard to conceive two identical humans if water, which constitutes seventy percent of the human body, is a different chemical altogether in Twin Earth. These are mere realist claims that do not hinder the grounding of Putnam’s arguments. They rest on the choice of element and could be overcome by choosing some element that does not form part of the human body.

Where do you think meanings are then?


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