I suspect my mind of taking its observations of a person’s physical energy and dexterity as strong evidence about their mental quickness and clarity. The existence and the wrongness of this presumption were brought into relief for me by reading Jean-Dominique Bauby’s The Diving Bell and the Butterfly, on his life with locked-in syndrome. Because realizing that the author’s lively and intelligent voice was issued from a single blinking eye looking out of a mostly inert body felt like seeing a mag...