A on sofas:

Tyler Burge (Burge (1979)) has developed a very influential line of anti-individualistic thought. He argued that the cognitive content of a person’s concepts depends in part on their socio-linguistic environment. The argument of that paper centered around the example of a subject, now often called ‘Alf’, who does not know that by definition ‘arthritis’ applies only to conditions of the joints. We are to imagine that Alf has many typical, mundane beliefs about arthritis: he believes that arthritis is a painful condition, that he himself has suffered from arthritis for years, that his arthritis in his wrist and fingers is worse than his arthritis in his ankles, and so on. One day he wakes up with a new pain and fears that his arthritis has spread to his thigh. On a twin earth, the term ‘arthritis’ is used more liberally than it is here, and applies to various rheumatoid conditions of bones and other sorts of tissue, and indeed is true of the condition that Alf has in his thigh. Twin Alf, being an inhabitant of this twin earth, has no concept of arthritis and so does not believe that his arthritis has spread to his thigh.