Social externalism is a thesis about the individuation-conditions of thoughts. Actually, the thesis applies only to a special category of ‘trained’ thoughts, thoughts which issue from trained thinking. It isn't that the thinker of such a thought has to have had special training about the subject-matter. It is rather that he or she needs to have acquired certain basic linguistic skills and values. For trained thoughts are thoughts whose contents are tailored to the demands of communication. Social externalism, as I understand it, says that people who are competent in a public language are equipped to have certain thoughts whose contents are fixed (in part) by the lexical semantic norms of their language.
Andrew Woodfield is Reader in Philosophy at Bristol University