It is natural to distinguish, for any thinking creature, those events and states that are internal to the creature -- its brain states, for example -- from those that are not. Narrow mental content, if there is such a thing, is content that is entirely determined by the goings-on inside the head of the thinker. A central question in the philosophy of mind since the mid-1970s has been whether there is a kind of mental content that is narrow in this...