Sacks wrote earlier that neurology’s favorite word is “deficit.” Neurologists have historically studied patients who lack certain functionality, but they have rarely studied patients with the opposite condition—excess. Sacks believes conditions of excess can teach us just as much as conditions of lack.

In Part 2, Sacks turns his attention to conditions of excessive functionality. Whereas conditions of deficits often present as a lack of ability, conditions of excess manifest in unusual or atypical actions and behaviors. These can lead to increased creativity but can also lead to negative side effects: “The paradox of an illness which can present as wellness—as a wonderful feeling of health and well-being, and only later reveal its malignant potentials—is one of the chimeras, tricks and ironies of nature” (44). He gives the example of author George Eliot (Mary Ann Evans), who reported that she felt “dangerously well” before a migraine, or Sacks’s own patient, Rose R. of Chapter 16, who felt “the joy of restored health” (47) but knew that the feeling would not last. Sacks explains that people with “excess” conditions might feel as though they have a heightened sense of knowing before tipping over into elevated paranoia. In the following part, Sacks relates tales of patients who demonstrated some of these conditions of excess.