Twin Earth and Semantic Externalism

Hello! Welcome to your next lesson.

In our last session, we explored Kripke's causal-historical theory of reference. We saw how it provides an alternative to descriptivism, suggesting that names and natural kind terms get their reference through an initial "baptism" and a subsequent chain of communication. This led us to a provocative conclusion: the meaning of our words seems to depend not just on our internal thoughts or beliefs, but on our causal connections to the external world. This view is known as semantic externalism.

Today, we will dive headfirst into this radical idea. We'll examine one of the most famous and ingenious thought experiments in all of analytic philosophy: Hilary Putnam's Twin Earth argument. By the end of this lesson, you will be able to explain this thought experiment and its powerful case for semantic externalism, fulfilling this lesson's learning outcome. Putnam was a contemporary of Kripke, and their work on these topics is deeply intertwined, together forming a major turning point in the philosophy of language.

1. The View From Inside: Semantic Internalism

Before we travel to Twin Earth, we need to understand the view that Putnam was arguing against, which we can call semantic internalism. Traditionally, philosophers believed that the meaning of a word was something fully grasped by the speaker's mind.

This traditional picture rests on two core assumptions. Let's watch a short video that lays them out clearly.

As the video explains, the two assumptions are:

1. Psychological Component: To know the meaning of a term (e.g., 'water') is to be in a certain psychological state. This state includes your concepts, beliefs, and mental images associated with the term. We can call this the "meaning is in the head" thesis.
2. Semantic Component: The meaning of a term (its intension) determines what it refers to in the world (its extension).

Given your background in mathematics, you can see the logical implication here: If psychological state determines intension, and intension determines extension, then the psychological state of a speaker must ultimately determine the extension of their terms.
$\text{Psychological State}\to \text{Intension (Meaning)}\to \text{Extension (Reference)}$
Putnam's goal is to show that this chain of reasoning is broken. He argues that two speakers can be in the exact same psychological state, yet the extension of their terms can be different.

In a lecture from 2011, Putnam himself described his dissatisfaction with this internalist picture, which led him to this groundbreaking work.

2. The Twin Earth Thought Experiment

Putnam's conclusion that "meanings aren't in the head" is most powerfully supported by his Twin Earth thought experiment. The setup is designed with surgical precision to isolate the variables of psychological state and external environment.

This image provides a quick summary of the premises.

Now, let's see the thought experiment in action.

To complement this, let's hear the argument from Putnam himself. He adds some interesting clarifications about the thought experiment's design.

Finally, for a concise written version that you can refer back to, let's turn to the Stanford Encyclopedia of Philosophy. This entry also makes the crucial move from the meaning of words to the content of our thoughts.

Let's formalize the argument's structure:

1. Premise 1: On Earth in 1750, the term 'water' refers to H₂O. (This follows from the causal-historical theory we saw last lesson: the term was "baptized" to refer to this stuff, which, as a matter of scientific fact, is H₂O).
2. Premise 2: On Twin Earth in 1750, the term 'water' refers to XYZ. (For the same reason, but in a different environment).
3. Premise 3: An Earthling (Oscar) and his Twin Earth duplicate (Twin Oscar) in 1750 are molecule-for-molecule identical. Their brains are in the same state, and they have had identical life experiences. Therefore, they are in the same narrow psychological state. All their beliefs, concepts, and mental images about 'water' are identical.
4. Conclusion: Oscar and Twin Oscar are in the same psychological state, but the term 'water' in their respective languages has a different extension (it refers to different substances).
5. Final Implication: Therefore, the psychological state of a speaker does not, by itself, determine the extension of their terms. The initial chain of reasoning (Psychological State → Intension → Extension) is broken.

3. The Externalist Conclusion: Division of Labor

Since the premise Psychological State → Extension is false, and Putnam wants to preserve the link that Intension → Extension, he must reject the first assumption: that knowing a meaning is purely a psychological state.

Meanings are not solely "in the head." They are determined in part by the external environment.

Putnam provides a second, complementary argument for this conclusion, which he calls the division of linguistic labor. This argument shows that even within a single community, meaning is not an individual possession.

The division of labor shows that meaning is social. I can refer to elm trees without being able to distinguish them from beech trees because I rely on experts (dendrologists) in my community to fix the reference. My meaning is partly determined by my social environment.

However, the Twin Earth case is more fundamental. In 1750, the entire linguistic community on Earth is in the same collective psychological state regarding 'water' as the entire community on Twin Earth. There are no experts on either planet who know about H₂O or XYZ. Yet, their words refer to different things. This shows that meaning is determined not just by our social environment, but by our physical environment as well.

Conclusion

Today, we have explored Hilary Putnam's seminal argument for semantic externalism. The Twin Earth thought experiment is a masterful piece of philosophical analysis, designed to show that our internal mental world is not sufficient to determine the meaning of our words.

Key Takeaways:

• Semantic Internalism is the traditional view that meaning is determined by psychological states "in the head."
• Putnam's Twin Earth thought experiment shows that two psychologically identical individuals can use the same word ('water') to refer to different substances (H₂O vs. XYZ) because they are in different physical environments.
• This leads to the conclusion of Semantic Externalism: the meaning and reference of our terms are determined, in part, by factors external to our minds, including our physical and social environments. As Putnam famously put it, "meanings just ain't in the head."
• The division of linguistic labor (e.g., the 'elm'/'beech' example) is a complementary argument showing our dependence on social context and experts to fix meaning.

Preview of the Next Lesson:

So far in this module, we have focused on semantics—the literal meaning of words and sentences. However, we often use language to communicate far more than what we literally say. In our next lesson, we will shift our focus to pragmatics by exploring H.P. Grice's theory of conversational implicature, which provides a systematic way to understand how we convey and interpret meaning in context.