"The Incubus" is a very early rational look at the concept of sleep paralysis and nightmares, with a bit of lore related to the night-stalking incubus (a demon associated with disturbing sleep.) With its herbal remedies and technically medical look at waking dreams and similar phenomena, it is at once technically outdated (scientifically) and for its era, forward-thinking.With some secondary sources (occasionally in Latin) mentioned and quoted, Wallers' work touches on a subject which remains debated to this very day.