Philosopher Tyler Burge (1946—) asks us to consider a case of a person who believes he has arthritis. Let us call him Jack, and let us say he lives in San Francisco. Jack has an inflamed thigh muscle. He goes to his doctor in San Francisco and says: “I have arthritis in my thigh.” The doctor looks at him and thinks it cannot be arthritis, because he has no joint issues. He says to Jack: “You have an inflammation in your thigh—not arthritis.” Now imagine there is a twin community, just like the one where Jack lives, but where they think of arthritis as inclusive of thigh inflammations. Twin Jack sees the twin doctor in Twin San Francisco. What will the doctor say when twin Jack says he has arthritis in his thigh? He might well say, “You are exactly right!”.
What is this supposed to show? In Putnam’s Twin Earth situation, what a person means is not up to the person. Burge’s case is similar. The belief of San Francisco Jack is false, while that of Twin San Francisco Jack is true. This leads Burge to think the beliefs are different because the same belief cannot be both true and false.
Since the beliefs are different, while the mental contents are type identical, we should conclude that beliefs are not in the head.
-  See Burge’s “Individualism and the Mental” in Chalmers (2002).