Hilary Putnam argues that firstly, meanings were originally thought to be a purely subjective phenomenon which was unique to an individual. Frege rebelled against this concept of meaning, thinking that meanings were a public affair. Secondly, that phrases such as ‘creature with a heart’ and ‘creature with a kidney’ may have the same extension, but not the same meaning.
The theory of meaning came to rest on two unchallenged assumptions:
1. Knowing the meaning of a term is just a matter of being in a certain psychological state (e.g. Beliefs, memories, etc.).
2. The meaning of a term determines its extension.
Are meanings all in the head?
In his twin earth thought experiment, Putnam imagines there being two earths which are identical in every way apart from what is referred to by the term ‘water’. On our earth, water is H20, while on the twin earth, water is a much more complex chemical compound called XYZ. If someone from our earth were to visit their earth, we would say they call water XYZ. But we would not call this water (the similar scenario occurs if they were to visit our world).
If we then went back to 1750, water would neither have the extension H20 or XYZ on both earths. If we had two people on their respective earths, Oscar 1 and Oscar 2, both of them would be in the same physiological state. Yet the essense of water being H20 in 1750, is as true as it being H2o in 1950. The same can be said for water as XYZ on earth 2.
For the sake of argument, Putnam asserts that Molybdenum and Aluminum pots and pans are indistinguishable. On our earth, Aluminum is plentiful, but molybdenum is rare. While on the second earth, its vice versa. The words for aluminium and molybdenum are switched on twin earth. If someone on our earth visited their earth, they would not suspect that what is called aluminium is actually molybdenum.
Someone trained however in prospecting metals, could tell what the metal actually is. While in 1750, no one could tell what the contents of water was. The confusion of metals involves confusions with the linguistic communities involved. The physiological state of a person does not determine its meaning.
A sociolingustic hypothesis
In language, there is a division of linguistic labor. There needs to be a way of coming to recognize things. In otherwords, our coming to know what a thing means is a mediate process in which we act, not an immediate state i.e. A psychological state, were what we mean is already there. Take the example of gold, it has different associations by the different way we interact with it, e.g. Gold rings, monetary item, an element, a status symbol, etc. There are much of activities which creates linguistic associations.
While a person can acquire the word gold, that person does not necessarily acquire how to tell if something is gold. To do this, that person must rely on a special subclass of speakers who can tell him (thier knowledge, coming about by their proffesions and occupations in the world). There is a joint community effort and division of tasks. Water on earth and twin earth during 1750 did not have a division of linguistic labor in this regard. Nor any experts which could diffientiate H20 and XYZ.