Twin earth

The twin earth is a thought experiment that goes back to the philosopher Hilary Putnam . It established the concept of semantic externalism . This states that the meaning of a word cannot be known a priori , but that further, external factors must be known. The term semantic externalism has not yet been introduced in the original form of this thought experiment.

situation

Let there be a twin earth that is similar to planet earth in every detail . Everything there is like on our earth: There are the same continents and countries , the same languages and a resident who speaks the language German on Earth speaks German on Gemini Earth . For every inhabitant of our earth ( Erdling , English earthling ) there is an inhabitant of the twin earth ( Zwillingserdling, English twin earthling ), whereby these two do not differ in anything. Like the earth, the twin earth is also largely covered with water and this is the basic building block of life there. The twin Earthlings, just like the Earthlings, refer to their water as water; H. with the word "water" from their respective language.

The only difference to our earth is the chemical structure of the water: While on our earth the sum formula applies, on the twin earth the formula applies . Just like is also colorless, odorless, wet and quenches thirst. The chemical structure is different, but the behavior is the same and the two waters are otherwise indistinguishable from each other. ${\ displaystyle H_ {2} O}$ ${\ displaystyle XYZ}$ ${\ displaystyle H_ {2} O}$ ${\ displaystyle XYZ}$

To carry out the thought experiment, Putnam presents two different viewpoints:

In the year 2050, astronauts from Earth will discover Twin Earth and find out the structure of water there. They spark back to earth that the term "water" denotes on twin earth . Conversely, astronauts from twin earth discover the earth and transmit back that "water" means there. ${\ displaystyle XYZ}$ ${\ displaystyle H_ {2} O}$
In 1750, the chemical structure of water was not yet known. In the case of an exchange between the inhabitants of the earth and the twin earth, the distinction would be impossible: The term "water" means two completely different substances, but nobody knows.

consequences

Both Earthlings and Twin Earthlings use the word "water" in the same way. On both planets it denotes a substance with the same external properties. Only this substance has the structure of on one planet , but on the other . It follows that, although both people mean the wet, colorless and odorless substance that quenches thirst by their water, the term on earth denotes a different substance than on twin earth. ${\ displaystyle H_ {2} O}$ ${\ displaystyle XYZ}$

Simplified, one can also imagine two completely identical people, one of whom (person A) lives on earth and the other (person B) on twin earth. When A and B each drink a glass of water, which they must do at exactly the same time and under the same circumstances, A thinks about it, but B thinks about it . ${\ displaystyle H_ {2} O}$ ${\ displaystyle XYZ}$

The only way to determine what a person means by the term “water” is to include their environment.

argumentation

Putnam makes the following premises :

(1) Knowing the meaning of a phrase is to be in a certain mental state.
(2) The meaning of an expression determines the extension .

From these two premises it follows:

(3) A certain psychological state determines the extension of the expression.

Putnam now shows, among other things, with the thought experiment of the twin earth that the conclusion (3) cannot apply, since in a completely identical psychological state a term can designate two different substances and thus has different extensions. Accordingly, the psychological state is insufficient to determine the meaning of a term and the conclusion (3) is untrue. This in turn means that assumption (1) or (2) must also be wrong. Putnam suggests rejecting assumption (1).

Counter arguments

Water can be seen as a cluster concept . Accordingly, the term “water” would not designate, but something in the form of the liquid, wet, thirst-quenching substance of which the oceans and lakes consist and which I can drink . The inhabitants of both earths would agree on this. This counter-argument can be reinforced by the fact that there are definitely people on earth who know this meaning of "water" but are not familiar with its chemical structure. One could argue that the chemical structure of water is in fact irrelevant to the meaning of the term “water”. ${\ displaystyle H_ {2} O}$

Thought experiments that are too different from reality could be misleading because they can only be intuitively understood in the context of actual reality.

The possibility of a twin earth, on which only a single element (here the structure of water) differs from its known form, seems doubtful, since each element has a multiple relationship with each other. There is the donkey bridge that the structure reminds of Mickey Mouse . This alone would no longer apply on Twin Earth. ${\ displaystyle H_ {2} O}$

Trivia

The films Another Earth and Accident in Space are about a journey to a second earth, which seems to be a perfect copy of ours. In both cases it does have small but crucial differences. In the event of an accident in space , the concept of counter-earth was also processed.

credentials

Putnam, Hilary (1975/1985): The meaning of 'meaning'. In Philosophical Papers, Vol. 2: Mind, Language and Reality. , Cambridge University Press.
Putnam, Hilary: The meaning of "meaning" , translated by Wolfgang Spohn , Klostermann texts philosophy.