Pausanias, Achaia, Chapter XVII. The ancient Babylonians believed in a certain "maid of the night," who appeared to men in sleep and roused without satisfying their passions. (Jastrow, Religion of Babylonia, p. 262.) This succubus was the Assyrian Liler, connected with the Hebrew Lilith. There was a corresponding incubus, "the little night man," who had nocturnal intercourse with women. (Cf. Ploss, Das Weib, 7th ed., pp. 521 et seq.) The succubus and the incubus (the latter being more common) were adopted by Christendom; St. Augustine (De Civitate Dei, Bk. XV, Ch. XXIII) said that the wicked assaults of sylvans and fauns, otherwise called incubi, on women, are so generally affirmed that it would be impudent to deny them. Incubi flourished in mediæval belief, and can scarcely, indeed, be said to be extinct even to-day. They have been studied by many authors; see, e.g., Dufour, Histoire de la Prostitution, vol. v, Ch. XXV, Saint-André, physician-in-ordinary to the French King, pointed out in 1725 that the incubus was a dream. It may be added that the belief in the succubus and incubus appears to be widespread. Thus, the West African Yorubas (according to A. B. Ellis) believe that erotic dreams are due to the god Elegbra, who, either as a male or a female, consorts with men and women in sleep.
"If any man's seed of copulation go out from him, then he shall bathe all his flesh in water and be unclean until the even. And every garment, and every skin, whereon is the seed of copulation, shall be washed with water and be unclean until the even." Leviticus, XV, v. 16-17.
It should be added that the term pollutio also covers voluntary effusion of semen outside copulation. (Debreyne, Mœchialogie, p. 8; for a full discussion of the opinions of theologians concerning nocturnal and diurnal pollutions, see the same author's Essai sur la Théologie Morale, pp. 100-149.)