Rambam - 1 Chapter a Day

I.e., not only did they teach the matter in theory, they actually had their conception applied in practice.

It would appear that this would apply even if the later court was lesser in wisdom and in the number of adherents than the court that made the original ruling (Radbaz).

Rosh HaShanah 25b states: “Would you think that a person would go to a judge that was not in his age? Instead, the intent is that he should follow the judge in his age.... Yerubaal in his generation is like Moses in his generation. Yiftach in his generation is like Samuel in his generation.” Although Yerubaal and Yiftach represented - to make an understatement - less than the epitome of wisdom and righteousness, a person in their generation was enjoined to follow the rulings of their courts.
The concept that a lesser court can challenge the rulings of a previous court of greater stature appears to be supported by the Rambam’s statements in his Introduction to the Mishneh Torah where he states that after the conclusion of the Talmud, the Geonim of one generation can challenge the rulings of the Geonim who preceded them. Nevertheless, it appears to be contradicted by his statements there that no later sage can challenge the rulings made by the Sages of the Talmud. Similarly, in the Talmud itself, it appears that the Sages of the Gemorah (the Amoraim) would not challenge the rulings of the Sages of the Mishnah. Thus it would seem that there were cut-off points at which the Sages of one generation would not challenge the rulings of the Sages of previous generations.
The Kessef Mishneh explains that it is possible to explain that, after the conclusion of the Mishnah, all the Sages accepted the general principle not to challenge the rulings of the Mishnah, and similarly, after the conclusion of the Talmud, not to challenge the rulings of the Talmud. When, however, there are no general principles of this nature in effect, the principle stated by the Rambam in this halachah applies. See also Hilchot Sanhedrin 6:1.

The Or Sameach relates that this principle explains why the Rambam calls the laws derived through the principles of Biblical exegesis, divrei sofrim, “the words of the Sages.” For their interpretation is dependent upon the Sages of each generation.

The Supreme Sanhedrin as indicated by the following halachah.

I.e., instituted a halachic practice on their own authority.

See Halachot 5-7.

In his Commentary to the Mishnah (Ediot 1:5), the Rambam interprets this term as meaning that the head of one court will be more renowned for his wisdom than the head of the other court.

The Ra’avad takes issue with the Rambam’s statements, maintaining that once a decree has gain widespread acceptance within the Jewish community, it may not be nullified even by the prophet Elijah and his court. The Ra’avad’s position is based on Avodah Zarah 36a which makes such a statement with regard to the 18 matters in which the School of Shammai’s opinion was accepted over that of the School of Hillel’s (see Shabbat 17a). The Radbaz and the Kessef Mishneh, however, explain that these 18 matters represent an exception to the rule (see also the notes to Halachah 3). With regard to other matters. The Rambam’s ruling is accepted.

Beitzah 5b derives this concept from God’s directives to the Jewish people before the Giving of the Torah. At first, He forbade sexual relations for three days in preparation for the Giving of the Torah. After the Giving of the Torah, there was a special command for the Jews to resume relations. Otherwise, although the reason for the safeguard had already been nullified - for the Torah had already been given - the original prohibition would have remained in effect.
The Ra’avad takes issue with this concept as well, stating that it. the rationale for an edict or decree has been nullified, the edict or decree can be nullified even if the court which does so is lesser than the court which instituted the decree. The Radbaz and the Kessef Mishneh, however, support the Rambam’s ruling.

We have been translating biminyan, lit. “in number,” as “in the number of adherents,” including the Rambam’s resolution in the translation of the term from the outset. Others (the Ra’avad in his commentary to Ediot 1:5) interpret “in number,” as “in age.’’

This ruling depends on the passage from Avodah Zarah 36a cited above. The Rambam maintains that the reason these 18 matters were given such power is because they were instituted as safeguards (Radbaz, Kessef Mishneh).

See the Chatam Sofer (Yoreh De’ah, Responsum 13) which states that this ruling applies only when the rationale for which the safeguard was originally instituted still applies. If, however, that rationale was nullified, a court that is of greater stature than the court which instituted the decree may nullify it.

I.e., although there are situations where we apply the principle (Ketubot 83b): “The words of the Sages are more severe than the words of the Torah,” this is not one of them (Kessef Mishneh).

I.e., even capital punishment as indicated by the following note.

In that vein, the Rambam states (Hilchot Sanhedrin 24:4, quoting Sanhedrin 46a): “An incident occurred where they had a man lashed for engaging in relations with his wife under a tree. And an incident occurred concerning a person who rode on a horse on the Sabbath in the era of the Greeks and they brought him to the court and had him stoned to death.”

They must, however, clarify that their instructions are given only because of the immediate situation and should not be adopted as a permanent practice. The classic example of this is Elijah’s confrontation of the prophets of Baal at Mount Carmel (I Kings, ch. 18). Although there is both a positive and a negative commandment that requires all sacrifices to be offered in the Temple, Elijah offered a sacrifice on Mt. Carmel to prove God’s supremacy to the people. In Hilchot Yesodei HaTorah 9:3, the Rambam discusses the issue within the context of the license granted a prophet to temporarily violate the Torah’s commandments, stating:
If [the people] would have asked Elijah: “How can we violate the Torah’s command,”... he would have told them: “...Anyone who offers a sacrifice outside [the Temple’s premises] is liable for karet... [The present instance,] however, [is an exception]. I am offering a sacrifice ... at God’s command to disprove the prophets of Baal.”
Just as a prophet is empowered to take such license based on God’s command, a court may take such license based on their own perception of the situation.

In his gloss to this halachah, the Radbaz writes:
The analogy is not appropriate unless we see the entire Jewish people as one body. Although their bodies separate them, since their souls are hewn out from a single source, they are like one body, for the soul is of primary importance. Know this. From our master’s words, it appears that the analogy applies to the mitzvot. They are like one body and the court may nullify some [of the mitzvot] so [the people] observe the rest. Both interpretations are correct.
According to the first interpretation, the intent is that we may punish some Jews (as quoted from Hilchot Sanhedrin) so that the observance of the people at large will remain intact. See also Berachot 54a which interprets Psalms 119:126: “It is a time to act for God; they have nullified Your Torah,” as “They [i.e., the Sages] may ‘nullify Your Torah,’ because ‘it is a time to act for God.’” The intent is that the sages may violate certain Torah commandments temporarily if they feel that doing so will allow the body of the Torah to remain intact.

Yoma 85b. See the Rambam’s statements in Hilchot Shabbat 2:1-3.

Avodah Zarah 36a derives this concept from the exegesis of a verse. It is, however, logically understood. Regardless of the Sages’ positive intent, instituting a practice which people at large cannot uphold will lead to a weakening - and not a strengthening – of Torah observance.

I.e., it is not necessary for a subsequent court to undertake a specific action to nullify the decree. If it was not accepted by the community, it is nullified automatically.
The Ramah (Yoreh De’ah 228:50, based on Piskei Mahari 292) rules that individuals also are not obligated to observe such decrees.

The Ramah (Yoreh De’ah 228:50, based on Piskei Mahari 292) rules that individuals also are not obligated to observe such decrees.

See the Kessef Mishneh who debates whether the intent is that the decree never spread throughout the Jewish community (as is Rashi’s view) or that at his time, its observance was not widespread. The latter interpretation would lead to the conclusion that even when originally, the Jewish people had by and large observed a decree, if it is not observed in a later generation, it can be nullified. The Kessef Mishneh does not, however, conclusively accept either interpretation.

The Rambam's wording implies that since the original court was under the impression that the decree had spread throughout the Jewish community, the later court must nullify the decree. It is not automatically null and void.
One of the classic examples of this concept is the actions of Rabbi Yehudah Nesiah (the grandson of Rabbi Yehudah HaNasi, the author of the Mishnah). Rabbi Yehudah and his court permitted the use of oil made by gentiles although the decree forbidding it had been issued by the students of Hillel and Shammai, a court of far greater prestige. Rabbi Yehudah surveyed the Jewish community and saw that the decree had never been observed. Therefore, he had the authority to nullify it.

Avodah Zarah 37a relates that after Rabbi Yehudah Nesiah repealed two prohibitions, some of his colleagues pressed him to repeal a third. He refused, saying that this would cause his court to develop a reputation for leniency.

As stated in Halachot 2 and 3.

As stated in Halachah 4.

Sefer HaMitzvot (negative commandment 313) and Sefer HaChinuch (mitzvah 454) count the prohibition against adding to the Torah as one of its 613 mitzvot.

Sefer HaMitzvot (negative commandment 314) and Sefer HaChinuch (mitzvah 455) count the prohibition against detracting from the Torah as one of its 613 mitzvot.

I.e., the interpretation of the Written Law given to Moses at Sinai - "the mitzvah" to refer to the term used by the Rambam in his Introduction to the Mishneh Torah – is considered as equivalent to the Written Law itself. Just as it is forbidden to add or detract from a concept explicitly stated in the Torah; so, too, it is forbidden to add or detract from the explanation of a practice conveyed by the Oral Law. The Rambam reiterates these concepts in the ninth of his Thirteen Principles of Faith (Commentary to the Mishnah, Sanhedrin, ch. 10) stating:
The ninth principle is that the Torah of Moses will never be nullified.... There can be no additions to it, nor any deletions from it - neither in its text nor in its explanation. And thus we are commanded: "Do not add to it and do not detract it from it."
(Our translation is taken from the original manuscript versions of the Rambam's Commentary to the Mishnah. The standard published text varies slightly.)
The Ra’avad harshly differs with the Rambam’s explanation, stating that the Rambam’s words are fit to be carried away by the wind. The Ra’avad states that any practice established as a safeguard for Scriptural Law is never considered as a prohibited addition to the Torah even if it is established as a permanent practice and an allusion to it from the Torah is found. According to him, addition and detractions from the Torah involve adding or detracting from the observance of certain mitzvot: e.g., wearing five or three tzitzit instead of the required four, or adding or subtracting a species to the four used on Sukkos. The Radbaz and the Kessef Mishneh support the Rambam’s conception, while Rav Moshe HaCohen echoes the Ra’avad’s view.

E.g., a deer or a buffalo which are kosher animals. The Rambam’s statements have attracted the attention of the commentaries, for there is a difference of opinion among the Rabbis in Chullin 116a, and the Rambam (Hilchot Ma’achalot Assurot 9:4) and many other Rishonim follow the view that the meat of a wild beast that is cooked in milk is not forbidden according to Scriptural Law. Why then does he state here that it is forbidden?
The Radbaz and the Kessef Mishneh explain that here, the Rambam is speaking theoretically: Were the halachah to follow the opinion that the meat of a wild beast is forbidden according to Scriptural law, the ruling would be such and such. The Merkevat HaMishneh, however, maintains that a printing error crept into the text in Hilchot Ma’achalot Assurot and the text should be changed to fit the Rambam’s ruling here.

For the verse mentions “its mother’s milk,” and fowl do not have milk (Chulin 113a).

This and the subsequent conclusions the Rambam mentions are all forbidden by Scriptural Law.

For the verse mentions explicitly a "kid," i.e., a goat.

I.e., interpreting the term according to its most literal meaning.