The sei is the Kaluli witch, a malevolent kind of being whose hidden aspect preys upon others in the unseen world. Acting invisibly, a sei seizes a victim's reflected body, dismembers it, and consumes its substance, so that the living person weakens and, untreated, dies. Healing is the work of a spirit medium, who leaves his sleeping body to hunt down the scattered parts, wash their decay away in the invisible river Sili, and warm them back to wholeness. The sei thus supplies the Kaluli account of grave illness and untimely death, and the medium's nocturnal recovery of the body stands opposite it as the principal means of cure.