The word "incubus" is derived from the Latin incubare, meaning to lie down on. The word "succubus" is derived from the Latin succubare, to lie under. In Latin and then in English the words referred respectively to male and female demons believed to have sexual intercourse with humans. Because Jones interprets the nightmare as a symptom of pathologically repressed sexuality, he identifies the incubus as the same experience with a more manifest sexual content. The same assumptions about the nature of all dreams and the "meaning" of all fear in dreams that led him to discount the difference between sleeping nightmares and waking nightmares led him to discount the phenomenological differences between purely terrifying accounts and erotic accounts. This dogmatic insistence that latent sexuality is involved in all frightening dreams has been criticized by many recent writers.22

The presence of overt sexual content as a frequent feature of the nightmare proper is also highly debatable. I have encountered a few explicit sexual details in Old Hag accounts, but these are rare and are not typically major components of a given experience. Some overtly sexual accounts have appeared in the course of my investigation, but these differed from the Old Hag in that they have lacked the paralysis feature and, in several cases, fear. These probably constitute either a distinct subtype of the experience or a different phenomenon altogether. Some of the instances Jones viewed as manifesting sexual content probably were in fact REM dreams, and erotic dreams, with or without orgasm, are a well-known phenomenon. It is possible that there are also erotic experiences of this kind that subjectively appear to occur in a waking state. Such events do not seem to have been reported in the literature, but this may be a result of the typically broad connotations of the word "nightmare," after Jones. At any rate, it would be a mistake automatically to assume that no more realistic experience lies behind the widespread incubus traditions. To do so would invite a repetition of the errors and confusion that have characterized writings about the Old Hag variety of experience.

From ancient times, it seems to have been recognized that at least two different kinds of experience were involved in traditions of supernatural contact related to sleep. Some terms for these experiences have been used only in a restricted sense, referring either to sexual encounters or to terrifying attacks with restraint and pressure. Others were used in a more general way and could cover either or both kinds of experience. In addition to the Latin "incubus," for example, there were the Greek words Ephialtes ("leaper"), Pnigalion ("throttler"), and the Latin Inuus ("one who sits on").23 The reference to pressure as a primary feature is especially clear in Pliny's use of suppressio nocturna for the nightmare.24