Concurrent with the release of the new Ghostbusters movie, somebody asked me about Judaism's position vis-a-vis ghosts. The truth is that the term "ghost" is somewhat vague. The ghosts in Ghostbusters are a mix of the spirits of dead people, demonic entities, and strange slimey things.
In various Jewish texts, the spirits of dead people are described as appearing in terms of gilgulim and dybbuks. Rabbi Reuven Margolies, for example, explains that the reason why a murderous ox is put on a human-like trial is that it houses the reincarnated spirit of a person. Belief in such gilgulim, first discussed in the era of the Geonim, is found with many authorities, though it was also rejected by many others.
(Opponents to the belief in gilgulim include Rav Saadiah Gaon, Emunos v’Dayos 6:8; Rabbeinu Avraham ben HaRambam (see R. Margolies, in his introduction to Milchamos Hashem p. 19 note 11); Rabbi Avraham ibn Daud, in Emunah Ramah 7; Rabbeinu Yitzchak ben Avraham Ibn Latif, Rav Poalim, p. 9 section 21; Rav Chasdai Crescas, Ohr Hashem, ma’amar 4, derash 7; Rav Yosef Albo, Sefer HaIkkarim 4:29; and Rav Avraham Bedersi, Ktav Hitnatzlut leRashba. See too Rashash to Bava Metzia 107a. Also see Rabbi Samson Raphael Hirsch, commentary to Genesis 50:2. For further discussion, see Rabbi Yitzchak Blau, “Body And Soul: Tehiyyat ha-Metim and Gilgulim in Medieval and Modern Philosophy,” The Torah u-Madda Journal vol. 10, 2001.)
A much later belief is that in dybbuks - malevolent spirits that take over a person's body. One of the most famous such accounts is that of Rav Elchonon Wasserman, who told about an exorcism performed by the Chafetz Chaim. However, in Making Of A Gadol, Rav Nosson Kamenetzky cites various members of the Chafetz Chaim's family who said that the woman concerned was mentally ill, and the Chafetz Chaim was simply catering to her beliefs.
Neither gilguls nor dybbuks are found in classical rabbinic texts such as the Talmud or Midrash. However, there are countless references in these texts to sheidim and mazikin. What are these? In a very strange column published yesterday in Hamodia/ Cross-Currents, Rabbi Avi Shafran quotes the statement of the Talmudic sage Abba Binyamin that "Were the myriad mazikin that constantly surround us visible to us... we would be frozen in terror." Rabbi Shafran adds that "Whether he had in mind the fungi, protozoa, bacteria and viruses that regularly seek to invade our bodies must remain speculation."
Personally, I think that "wishful thinking" would be a better description than "speculation." It is, frankly, rather odd to posit that Chazal knew about such microscopic phenomena. First of all, considering how many very basic things about the natural world they did not know, such as the sun's path at night, why on earth would they have known about microscopic phenomena? Second, if they did indeed know about bacteria and viruses, then why on earth didn't they issue basic medical advice which would have saved countless lives over history, instead of all kinds of bizarre potions and procedures? Third, let's look at the full statement from the Gemara:
It has been taught: Abba Benjamin says, If the eye had the power to see the mazikin, no creature could endure them. Abaye says: They are more numerous than we are and they surround us like the ridge round a field. R. Huna says: Every one among us has a thousand on his left and ten thousand on his right. Rava says: They are responsible for the crushing in the Kallah lectures, fatigue in the knees, the wearing out of the clothes of the scholars from rubbing against them, and the bruising of the feet. If one wants to discover them, let him take sifted ashes and sprinkle around his bed, and in the morning he will see something like the footprints of a cock. If one wishes to see them, let him take the afterbirth of a black she-cat which is the offspring of a black she-cat, the firstborn of a firstborn, roast it in fire and grind it to powder, and then let him put some into his eye, and he will see them. Let him also pour it into an iron tube and seal it with an iron signet that they should not steal it from him, and let him also close his mouth, so that he should not come to harm. (Babylonian Talmud, Berachot 6a-b)That hardly sounds like a description of fungi, protozoa, bacteria and viruses!
The mazikin and sheidim of the Gemara are demons. Many rabbinic scholars over the centuries believed in the existence of such entities, in part due to the authority of the Talmud. Others, notably Rambam, dismissed the notion of demons.
There is a comprehensive discussion of all the different rabbinic views on demons in my monograph, Wrestling with Demons: A History of Rabbinic Attitudes to Demons. As I conclude in my study, the mere fact of someone ultimately accepting that demons exist does not at all necessarily mean that he is not a rationalist — it all depends upon the historical context.
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