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Externalism holds that the semantic content of a belief or utterance depends on facts external to the speaker's mind and body. Putnam's Twin Earth case shows that water-thoughts on Earth and XYZ-thoughts on Twin Earth have different contents despite identical neural states. This has profound implications for psychology, epistemology, and philosophy of mind regarding what determines thought content.
Reconstruct Putnam's Twin Earth scenario in detail: Earth has H2O in lakes and oceans; Twin Earth has XYZ that's indistinguishable to casual observation but chemically different. Both Oscar and Twin-Oscar have identical internal states, yet "water" refers to different substances. Study how this challenges internalism and supports externalism, then examine responses from internalists who resist the conclusion.
Before externalism, the natural assumption about meaning was internalist: the content of your thoughts and words is determined by what is in your head—your concepts, mental representations, and internal states. If two people are in identical internal states, they mean and think about the same things. This assumption feels obvious because from the inside, your thoughts seem fully your own mental business. Externalism challenges this assumption at its foundation, using a case specifically designed to show that identical internal states can be associated with different semantic contents.
From your work on Twin Earth and Putnam's externalism, you recall the scenario: Earth has H2O; Twin Earth has XYZ, indistinguishable in its observable properties but chemically different. In 1750, before anyone knew about molecular structure, Oscar on Earth and Twin-Oscar on Twin Earth are in identical psychological states when they use the word "water." Yet Putnam argues their words mean different things: Oscar's "water" refers to H2O, and Twin-Oscar's "water" refers to XYZ. Meaning is not in the head. The external environment is partly constitutive of what your words refer to.
This motivates the distinction between broad content and narrow content. Broad (or wide) content is externally determined: it is the actual referent of your terms, fixed by what you are causally connected to in your environment. Narrow content is whatever can be determined by internal states alone—the conceptual role your term plays in your reasoning, its inferential connections to other concepts. Oscar and Twin-Oscar share narrow content for "water" (it plays the same inferential role in their thinking) but differ in broad content (different substances are referred to). Semantic externalism is the claim that the philosophically significant notion of content—the one that fixes truth-conditions—is broad content.
The implications extend in several directions. For psychology, if content is externally constituted, then psychological explanations that appeal to content are not purely internal explanations—they essentially involve the environment the agent is embedded in. For self-knowledge, externalism might seem to threaten privileged access to our own thoughts: if I cannot distinguish H2O from XYZ, do I know what I mean? Externalists typically respond that privileged access to the fact that I am thinking about water is compatible with not knowing its chemical nature. For epistemology, externalism makes "water is H2O" an a posteriori discovery even though it is necessary once known—an unusual combination that Kripke's work on necessary a posteriori truths helps explain.
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