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Content externalism holds that the content of at least some mental states is partly determined by factors outside the individual's head. Putnam's Twin Earth thought experiment (1975) argues that two molecule-for-molecule identical thinkers can have different thoughts if their environments differ: an Earthling thinking 'water' thinks about H₂O, while her Twin Earth duplicate thinks about XYZ, even though their internal states are identical. Burge's arthritis case (1979) extends this to social factors: a person who misapplies the word 'arthritis' to a thigh ailment has thoughts about arthritis (the actual disease) because of how the community uses the term. Together, these arguments establish that meaning 'ain't in the head' — mental content is wide, determined partly by physical environment and linguistic community.
Work through the Twin Earth scenario carefully, noting exactly where the content of the thought differs and what fixes it. Then examine the internalist response: narrow content (a level of content shared by duplicates) might be what matters for psychological explanation. Key texts: Putnam's 'The Meaning of "Meaning"' and Burge's 'Individualism and the Mental.'
From your study of intentionality, you know that mental states are "about" things — beliefs, desires, and thoughts are directed at objects or states of affairs in the world. Representationalism extends this: mental states have content because they contain internal representations. The internalist pictures this as a self-contained system: whatever your brain contains fully determines what your thoughts are about. Content externalism directly challenges this picture, arguing that what you think about is partly fixed by factors outside your skull.
Putnam's Twin Earth thought experiment is the sharpest entry point. Imagine a planet identical to Earth in every observable way, except that the liquid called "water" there — which looks, tastes, and behaves like H₂O — has a completely different chemical structure, XYZ. Before chemistry was developed, an Earthling and her molecule-for-molecule identical Twin Earth duplicate are in exactly the same internal states. Both are thinking "water is wet." Yet the Earthling's thought is about H₂O, while the Twin Earthling's thought is about XYZ. Same internal state, different content. The conclusion: internal states alone cannot fix the content of thought. The physical environment — specifically, what the word "water" actually tracks in each world — partly determines what these thoughts are about.
Burge's arthritis case extends this to social facts rather than natural-kind facts. Suppose you believe you have arthritis in your thigh. Unbeknownst to you, arthritis only affects joints — thigh pain cannot be arthritis. Yet your thought "I have arthritis in my thigh" is still about arthritis, the actual disease, because that is how your linguistic community uses the term. Your internal representation may be slightly wrong in its extension, but the content is fixed by community practice. This is social externalism: the meanings of your terms — and thus the contents of your thoughts — are partly determined by how your linguistic community uses those terms.
The internalist response introduces a distinction between narrow content and wide content. Narrow content is whatever you and your Twin Earth duplicate share — something like "the watery stuff in my environment." Wide content is the full semantic content, which differs between you: you refer to H₂O, your twin refers to XYZ. Internalists argue that narrow content is what matters for psychological explanation, since two people in the same internal states should be predicted to behave the same way. Externalists counter that wide content is what matters for semantic evaluation — for determining whether your beliefs are true or false — and that psychological explanations are richer and more accurate when they can appeal to what a thinker's environment actually is. The dispute has consequences for whether cognitive science must be purely internal or whether it inherently requires reference to a thinker's world.
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