  The need for third-person empirical data gathered by external observers is perhaps most obvious with regard to the more clearly functional types of consciousness such as access consciousness, but it is required even with regard to phenomenal and qualitative consciousness. For example, deficit studies that correlate various neural and functional sites of damage with abnormalities of conscious experience can make us aware of aspects of phenomenal structure that escape our normal introspective awareness. As such case studies show, things can come apart in experience that seem inseparably unified or singular from our normal first-person point of view (Sacks 1985, Shallice 1988, Farah 1995).