Roscher’s thesis, briefly is that the nightmare demon in antiquity is the great god Pan in any of his several forms, and that the experience of the nightmare demon then was similar to that reported in the psychiatry and psychology of Roscher’s own day.
This passage states the governing thesis of the entire volume: Pan is the archetypal substrate of the nightmare, linking ancient demonology to modern psychopathological experience.
, Pan and the Nightmare, 1972thesis