Twin Earth, Again

David Sanson

2026-09-30

Reminders

  • Second writing day is next Wednesday.
  • If you got a C or lower on the first writing assignment, you can improve it to a B by meeting with me and showing that you (now) understand how Frege and Russell would respond to the case you wrote about.
  • What do you want to do on Monday of next week?

Examples

  1. Oscar1 says ‘Water is wet’. True or false?
  2. Oscar2 says ‘Water is wet’. True or false?
  3. Have they said the same thing?
  4. Oscar1 says ‘Water is H2O’. True or false?
  5. Oscar2 says ‘Water is XYZ’. True or false?
  6. Have they said the same thing?

Externalism?

  • Our word ‘water’ means H2O because that is the stuff we causally interact with.
  • The Twin Earth word ‘water’ means XYZ because that is the stuff they causally interact with.
  • The meaning of the word is determined by something external to the psychological states of Oscar1 and Oscar2.

The Causal Historical View

‘Oscar1’ refers to Oscar1, not Oscar2. How?

  • Someone on Earth said, “I hereby name this baby, ‘Oscar’.” This introduced the name ‘Oscar’ into our Earth language, and fixed Oscar1 as its referent.
  • Someone on Twin Earth said, “I hereby name this baby, ‘Oscar’.” This introduce the name ‘Oscar’ into the Twin Earth language, and fixed Oscar2 as its referent.
  • Since then, those names have been passed down to other speakers, who continue to use them to refer to Oscar1 or Oscar2, respectively, even if they are not able to provide any unique description specifying the referent.

Likewise:

  • Someone on Earth said, “I hereby dub this kind of stuff, ‘water’.” This introduced the word ‘water’ into our Earth language, and fixed its extension as the kind of stuff they were pointing to, which (whether they knew it or not), was H2O.
  • Someone on Twin Earth did the same, fixing the extension of ‘water’ in their language to the kind of stuff they were pointing to, XYZ.
  • Since then, those words have been passed down to other speakers, who continue to use them to refer to H2O and XYZ, respectively, even if they don’t know any chemistry.

‘Beech’ and ‘Elm’

  • I say, “Elm trees are deciduous”. True or false?
  • I say, “Beech trees are deciduous”. True or false?
  • Have I said the same thing both times?
  • I say, “An elm tree fell on my house,” but it was actually a beech tree. True or false?

Division of Labor

Putnam gives the example of gold: you don’t have to know how to distinguish gold from fool’s gold to buy gold jewelry. There is a division of epistemic labor here: what you need to know as a consumer is different from what the seller needs to know, which is different from what the metallurgist needs to know. The social practice as a whole depends on all of these things together.

Can you come up with other examples of this?

Do agree with Putnam that this extends to language: our ability to use the word ‘gold’ to refer to gold and not fool’s gold, or ‘aluminum’ to refer to aluminum not molybdenum, or ‘elm’ to refer to an elm tree not a beech tree, might depend on the existence of experts who are able to distinguish these things?

Rigidity and Indexicality

A rigid designator designates the same thing in every possible world.

But there are various ways to achieve this!

Demonstrative
Let ‘Aristotle’ refer to that man. (No unique description ever provided)
Let ‘water’ refer to this stuff.
Reference-Fixing Description
Let ‘Denny’ refer to the person who invented donuts (whoever that might be).
Let ‘water’ refer to the liquid stuff that fills the rivers and oceans. (?)
Indexical Description
‘Aristotle’ means: the man who actually taught Alexander.
‘Water’ means: the liquid that fills the rivers and oceans around here.

“Narrow content” and “Broad content”

Maybe we can distinguish the kind of meaning that Oscar1 and Oscar2 have in common—call that “narrow content”—and the kind that they disagree on—call that “broad content”?

Mental Content Too?

What about the psychological states themselves? Putnam says that Oscar1 and Oscar2 are in the same psychological state. But Oscar1 believes that water is wet, while Oscar2 believes that twater is wet. Aren’t those different beliefs?

Putnam’s Argument

The traditional theory of meaning assumes:

  1. That knowing the meaning of a term is just a matter of being in a certain psychological state.
  2. That the meaning of a term determines its extension (in the sense that sameness of intension entails sameness of extension).

There is no such thing as meaning that satisfies both (1) and (2).