Rabbinic legislation: takkanot and gezeirot
Gezeira d’rabanan. A rabbinic fence law, aimed at deterring one from doing something that is prohibited.
For example, one may not place food directly on a fire before Shabbat in order to keep it heated during Shabbat. This is a fence around the law against cooking on Shabbat. To prevent the gezeira from being violated, a metal cover, called a blech in Yiddish, is placed on the stove top before Shabbat with the flame (turned to a low setting) under one section and the pot with food placed on the blech. This blech serves as a fence, allowing heating of the food without any danger of violating the law. Note that a “gezeira dirabanan” becomes binding only if it is accepted by the community. From the Soc.Culture.Jewish.Faq
A Takanah is rabbinic legislation. The term often refers to directives aimed at imposing a duty to perform a particular, act. Some are rulings not to engage in particular acts e.g. the 2 famous takkanot of Rabbeinu Gershom (c. 1000 CE – not marrying more than one woman, not opening another’s mail).
The Mishnah says that once a law (takknakah or gezeirah) is legislated by the proper authorities and accepted by the Jewish community at large, it can never be rescinded, with the exception that “One Bet Din cannot annul the decree of another Bet Din unless it is greater in both wisdom and numbers.” (Avodah Zarah 36a, and Eduyot 1:4).
Many people assume that all later courts are to be judged inferior to the earlier ones, based on the following Talmudic proverb: “If the earlier scholars were sons of angels, we are sons of men. If the earlier scholars were sons of men, we are like asses.”
– Talmud Bavli, Shabbat 112b; Talmud Yerusahlmi Demai 1:3, 21d; Talmud Yerushalmi Shekalim 5:1, 48c-d.
As Rabbi David Golinkin points out, however, this proverb refers only to self-sacrifice in the observance of mitzvot. In legal matters, our sages had the opposite approach, as expressed by Rabbi Dosa ben Hyrcanus in the Mishnah:
“Then Moses and Aaron, Nadav and Avihu, and seventy eldars of Israel ascended” (Exodus 24:9) And why were the names of the eldars not listed? To teach that every three [judges] who have served as court of law are equal in authority to the court of Moses. [Rosh HaShanah 2:9]
The Tosefta adds: “The court of Yerubal was as great in the eyes of God as the court of Moses. The court of Yiftah was as great in the eyes of God as the court of Samuel. To teach you that whoever is appointed a leader over the community – even the most worthless – must be considered like the mightiest of the mighty.”
– Rosh HaShanah 1:18, Lieberman edition, p.311-312, and cf. Bavli Rosh HaShanah 25b.
Are Mishnaic and Talmudic laws always binding?
No. Many Mishnaic laws are not accepted by the Talmud; similarly, many Talmudic rulings never were codified in later codes of Jewish law.
Examples of changing halakhah
Modern Orthodox Rabbi Mendell Lewittes writes about the ways that the classical sages modified or changed halakhah.
“the Sages themselves found the key to unlock the gates of burdensome restriction. Thus Rabbi Gamaliel and his Bet Din cancelled the pre-Sabbatical year restriction imposed by Bet Shammai and bet Hillel. When some Amoraim questioned this cancellation…the answer, given after a moment’s hesitation, was that those who made the gezerah originally did so on the condition that if a later authority sees fit to cancel it, they may do so. (Moed Katan 3b).” [Lewittes, p.96]
Here are 2 examples from R. Lewittes
1. “Another method of easing the burden was to limit the scope of its application. Thus Rava excluded a “noble person” from the gezerah against readin by the light of a lamp on Friday night since such a person does not adjust the light by himself even on a weekday. (Shabbat 12b and Rashi ad loc.)” [Lewittes, p.96]
2. This rule was construed as applying to a court only in its exercise of actually legislating the law, however later courts were allowed to interpret this law, and through so doing could thus arrive at a different conclusion through an alternative interpretation of a biblical passage or ancient halakhah. [Lewittes, referencing Mishneh Torah, Mamrim 2:1]
Here are more examples from the Encyclopedia Judaica, articles on Takkanah. Halakhah can be changed …
(1) If at the time of making its enactment, the court expressly prescribed that it could be annuled by any court wishing to do so. (Talmud Bavli, Moed Katan 3b)
(2) When an enactment once believed to have spread among all of Jewry is later found not to have spread among the majority of people. (Mishneh Torah, Mamrim 2:7).
(3) Whenever the original reason and justification for the enactment ceases to be valid. (Bezah 5a, 5b; Hassagot Rabad on Mishneh Torah, Mamrim 2:2, by Abraham ben David of Posquieres).
(4) “If circumstances require that it is seeming for the bet din to uproot even such matters (enactments and decress of other courts) – even though it be of lower standing than the earlier bet din – so that such decrees shall not be of greater stringency than the laws of Torah itself, since even the laws of the Torah may be uprooted by any bet din as an emergency measure”. (Maimonides, Mishneh Torah, Mamrim 2:4)
Encyclopedia Judaica, and Mendell Lewittes “Jewish Law: An Introduction”, Jason Aronson Judaica.