 The idea is that, for a wide range of judgements it is possible that one knows that something is F but wrongly supposes that a is F. That is, one has misidentified which thing is F. For other judgements, perhaps including the introspection based judgement that one has a headache, this sort of identification error is not possible. After Pryor’s (1999) influential discussion, this is typically known as immunity to which-misidentification, or wh-IEM.