In a lecture (transcribed by Curious Jew), Prof. J. J. Schacter said:
Question: How do you personally deal with the tension between halakha and history? If you found out that a minhag developed in a certain way, would you change it/ your practice of it based on how it was developed?
Answer: Never in a million years would I change it- I follow mesorah- bound to observe particular behavior- historic insight is fascinating but it will not affect my practice. I try not to let anything affect my practice. My core identity is as a talmid of the Mesorah. All kinds of issues- context of historical pursuits, but this does not affect my practice.
I'd like to focus on one practice that might to be impacted by historical research.
A Persian Custom in the Talmud:
According to the superstitious views of the Parsees, the paring of nails and cuttings or shavings of hair are unclean, and become weapons in the hands of the demons, unless they have been protected by certain rites and spells. They are withdrawn from his power by the recital of certain prayers, and by being deposited in the earth inside consecrated circles, which are drawn around them as an intrenchment against the fiend. (Cf. Darmesteter's Avesta', in the SBE., Vol. IV, Part I, pp. xcii, 186 sqq.)
This superstition is almost universal. Darmesteter points out parallels in the folklore of Bombay; among the Esthonians, on the shores of the Baltic; the Gauchos in the Chilian pampas, and in the Norse saga (vide loc. cit.). In the B. T. Niddah, 17a, we are told that among the five culpable venial sins is "the casting away of nails on the street." .... "Even though the parings are laid in a basket, tied and sealed, an evil spirit still rests upon them," etc. The explanation which follows this curious statement is even more quaint:
והנוטל צפרניו וזורקן לרשות הרבים: מפני שאשה מעוברת עוברת עליהן ומפלת ולא אמרן אלא דשקיל בגנוסטרי ולא אמרן אלא דשקיל דידיה ודכרעיה ולא אמרן אלא דלא גז מידי בתרייהו אבל גז מידי בתרייהו לית לן בה ולא היא לכולה מילתא חיישינן ת"ר ג' דברים נאמרו בצפרנים שורפן חסיד קוברן צדיק זורקן רשע
(See also Moed Katon, 18a [Wuensche's translation, Vol. I, p. 302; III, 176]; Kethuboth 76b; Gittin 70a; Kiddushin 41; Dr. G. Brecher, Das Transcendentale, Magie und magische Heilarten im Tal- mud [Vienna, 1850], pp. 178-9; Schorr's Hechalutz, II, p. 158; VII, 42, No. 13; Geiger's Zeitsch. f. Wissenschaft und Leben, IX, pp. 259-60.)
Pliny, in his Natural History, Vol. V, p. 285 (cf. Bohn's Engl. edition), likewise mentions the usages connected with the cutting of human nails. It is religiously believed by many, says he, that it is ominous, in a pecuniary point of view, for a person to pare his nails without speaking, on the market days of Rome [the "Nundinae" held every eighth day in Rome], or to begin at the forefinger in doing so: it is thought, too, to be a preventive of baldness and of headache to cut the hair on the seven- teenth and twenty-ninth days of the moon. (See also F. Nork, Sitten und Gebrauche der Deutschen, etc., Stuttgart, 1849, p. 514.)
The Jews were enjoined not to cut their hair or nails at new moon. This custom is commended especially to women. (See the sources mentioned in M. Briick's Rabbinische Ceremonialgebrauche, etc., Breslau, 1837, p. 76, n. 47.) Several interesting culture-historic superstitions may be found in R. Jehuda Chasid's Sefer Chasidim.
The talmudic reference, quoted above, is also mentioned by Abudraham (see Geiger's Zeitschrift, loc. cit., p. 259). The Parsic parallels to the above may be found in Darmesteter's Zendavesta, I, pp. 185-9: ".... which is the most deadly dead whereby a man increaseth most the baleful strength of the Daevas . . . ? Ahurah Mazda answered: 'It is when a man here below combing his hair or shaving it off, or paring off his nails, drop them in a hole or in a crack [?] .... Therefore, 0 Zarathustra! whenever here below thou shalt comb thy hair or shave it off, or pare thy nails .... thou shalt draw three furrows with a knife of metal around the hole, or six furrows or nine. ... .For the nails, thou shalt dig a hole, out of the house, as deep as the top joint of the little finger; thou shalt take the nails down there and thou shalt say aloud these fiend-smiting words: 'The words that are heard from the pious in holiness and good thought,"' etc. See above: חסיד קוברן and the entire quoted text.
The American Journal of Semitic Languages and Literatures, Vol. 15, No. 1,. A. Kohut (author of Aruch Ha-Shalom)
R' Yaakov Elman has an interesting shiur relating to the subject (See here). He notes that the statement of R' Yosef (Megillah 11a):
וארו חיוה אחרי תניינה דמיה לדוב ותני רב יוסף אלו פרסיים שאוכלין ושותין כדוב ומסורבלין בשר כדוב ומגדלין שער כדוב ואין להם מנוחה כדוב
is a play on words. R' Yosef is actually referring to the Daevas mentioning above (from here we see that the Amoraim had a Lithuanian pronounciation ;). Thus - "they grow hair like a Daev" hints to the practice of the Persians not to cut any hair. (The eatig and drinking is part of another Gemara (Berachot) that describes the charasteristics of Demons (that they eat and drink, etc.).
Now, it would seem strange that on the one hand R' Yosef should make fun of this practice of the Persians and at the same time a part of this practice should be mentioned as obligatory elsewhere. Further, the practice is attributed to R' Shimon B. Yochai who lived much bfore the Persians. Lastly, the reason is different then that of the Persians since the Persians are afraid of sending power to demons and the Talmud is worried about the effect on pregnant woman.
[Prof. Secunda commented: "There is an almost direct parallel to the pregnant women issue as well in the Middle Persian work, Shayest ne Shayest." So my last objection is to be ignored. I serarched but couldn't find the quote in the Shayest ne Shayest.]
The custom is also cited in Moed Katon (18a):
אמר רב שמן בר אבא הוה קאימנא קמיה דרבי יוחנן בי מדרשא בחולו של מועד ושקלינהו לטופריה בשיניה וזרקינהו שמע מינה תלת שמע מינה מותר ליטול צפרנים בחולו של מועד ושמע מינה אין בהן משום מיאוס ושמע מינה מותר לזורקן איני והתניא שלשה דברים נאמרו בצפרנים הקוברן צדיק שורפן חסיד זורקן רשע טעמא מאי שמא תעבור עליהן אשה עוברה ותפיל אשה בי מדרשא לא שכיחא וכי תימא זימנין דמיכנשי להו ושדי להו אבראי כיון דאשתני אשתני
Now the subject of nails is brought up several times in the Zandavesta (from http://www.avesta.org/).
Kohuts reference is the Sar Dur Ch. 14. Here is another reference:
Regarding the bird Ashozusht, which is the bird Zobara-vahman and also the bird Shok, they say that it has given an Avesta with its tongue; when it speaks the demons tremble at it and take nothing away there; a nail-paring, when it is not prayed over (afsud), the demons and wizards seize, and like an arrow it shoots at and kills that bird. 20. On this account the bird seizes and devours a nail-paring when it is prayed over, so that the demons may not control its use; when it is not prayed over it does not devour it, and the demons are able to commit an offense with it.
At all events, there are important differences between the Babylonian Talmud and Zoroastrianism in this subject. To wit: reason (pregnant woman\demons), I found no source in the Zand Avesta that changing its place will cause a loss of power (but the Zand Avesta was originally oral so it may have been left out?), the business with the bird certainly has no connection with the Talmud.
Most intriguing is that the words Chassid Kovran is mentioned as part of a Baraita. Is this a "later" Baraita, or is this one of S. Y. Friedman's creatively redacted Baraitot?
There is an important Nafka Mina - Halacha L'Maaseh from this piece of Zand-Avesta since according to them only a pious person can bury the nails since he can say "words of piousness" a regular person would have to burn it.
I imagine source criticism might help solve part of the question since one might say that the original Halacha was a distinctly Jewish custom (in the ties of Rasbi), that was later conflated with the Zoroastrian over the course of the Galut. In any event, I do have to say that given the obvious Zoroastrian origin of several customs, I find Prof. Shacter's response rather insufficient.