The Twin Earth Argument

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This article is an introduction to the Twin Earth Argument.
The Twin Earth Argument was proposed by Putnam to prove that the meaning of language is not an internal or mental object.




Meaning and mental objects

The meaning of language is one important project in the philosophy of language. Philosophers often wonder why some of our voices have meaning and some don't. How and in what ways through which a voice conveys meaning are the two questions that philosophers always face.

In the history of philosophy, some people embraced the so called "Mental Object Theory" (or "Idea Theory," in Wiki's terms.) They considered "meaning" to be internal, or mental objects. In philosophy, internal or mental objects mean something that exists in the mind, such as ideas, beliefs, emotions and feelings.

Mental objects in the mind may change. For example, I was in a bad mood this morning before breakfast because I didn't want to wake up, but after breakfast I felt better. In this case we can say that there were different mental objects in my mind before and after breakfast this morning.

We usually use the phrase "mental state" to address a mind with a particular mental object at a particular time. Therefore, we can also say that I had different mental states before and after breakfast this morning.

People who consider meaning to be a kind of mental object would accept the following two points:

1. The meaning of a meaningful sentence is just a mental object that the speaker wants to convey.
2. To understand a meaningful sentence is to understand the mental object that the speaker wants to express.

However, Mental Object Theory is precisely what Putnam would reject.

The Twin Earth Argument

Far, far away in the Universe, there is a planet named "Twin Earth."
The discovery of Twin Earth made NASA investigators excited. However, they had some worries about its possibly malignant inhabitants, so they sent Agent Little Jones to the planet in order to do some secret research.

After being on the planet for a few days, Little Jones found out that Twin Earth is just like the Earth: on Twin Earth there is a blue sky, and there are also green trees, dogs, cats, Republicans, Democrats, Starbucks and McDonald's.

However, despite the surprising resemblance, there is one difference between the Earth and Twin Earth: on Twin Earth, the transparent and tasteless liquid in streams, rivers and seas, is composed of the molecules XYZ instead of H2O. Little Jones also discovered that given the progressing science and civilization just as on the Earth, people on Twin Earth have no ideas of any molecule.

In other words, Twin Earth residents don't know that the composing element of Twin Earth water is XYZ, nor do they know the composing element of Earth water is H2O (in fact, they have no knowledge of the Earth's existence). For the inhabitants on Twin Earth, water is just a "transparent, tasteless and salt-dissolving" liquid which expands at 4 degrees Celsius.

Twin Earth residents don't know their drinking water's composition, just like some people on Earth- one of them being Big Jones, father of Little Jones. Because of his lack of education, Big Jones doesn't know that his drinking water is composed of the molecules H2O. For Big Jones, water is just a transparent, tasteless and salt-dissolving liquid that expands at 4 degrees Celsius.

Therefore, we can say the Big Jones' understanding about water is equal to that of the Twin Earth residents. Therefore, when Big Jones thinks about water on Earth, he has the exact same mental object with those of the Twin Earth residents when they think about water on Twin Earth.

Therefore, according to the "Idea Theories," whenever Big Jones and a Twin Earth resident say "water," they are expressing the same object, and thus the same meaning.

Nevertheless, Putname would consider their meanings different. Apparently, when a Twin Earth resident says "water," what he wants to express is the Twin Earth water, which is XYZ. However, when Big Jones says "water," what he meant is the Earth water H2O.

XYZ and H2O are actually two different things, so the words referring to them would have different meanings as well. Therefore Putnam said, according to this experiment, the "Idea Theories of Meaning" fail.

At this point, is Putnam's argument valid?
If not, what's wrong with it?
If yes, then what do sentences mean?