Laws which appear in the Jerusalem Talmud and are omitted from the Babylonian Talmud: Tos. Berachot 11b #2
We rule like a Mishnah against a Baraita: Succah 19b; Makkot 21a
Ruling like a Baraita against a Mishnah which expresses an individual's view: Succah 19b
Siding with an opinion which has a Baraita to go with it, even though there is an outstanding question against that opinion [Tiyuvta veHilchita!]: Eruvin 10a, 16b; Bava Kama 15b
Siding with an opinion which has a special argument in its favor, even though there is an outstanding question [Tiyuvta] against that opinion: Berachot 23b-24a
Not learning practical law from the Talmud: Niddah 7b
Citing an example of a great Rabbi practicing in a certain way, to bolster one's position: Eruvin 6a, 6b, 102a; Pesachim 100a; Kiddushin 39a-b
Where one has an anecdote to show Law, the anecdote has priority ["Maaseh Rav"]: Shabbat 126b
Rulings of the Mishnah are not to be expanded to include all cases, unless such an expansion is indicated in the Mishnah itself: Shabbat 148a
[Not] proving a law by citing the fact that it is commonly practiced in that way: Eruvin 14b; Succah 44a; Moed Katan 18b; Bava Metzia 15b
Source for basing Law on Rabbinic knowledge of Agriculture: Shabbat 85a, Rosh HaShanah 13a [2x]
Not bringing a proof based upon a minor's account: Megillah 20a
Not bringing a proof based upon a case in which a minority opinion practiced a certain view: Megillah 20a
Establishing practical law in accordance with one's own learning: Niddah 20a
A Rabbi may not say, "If the circumstances of my examination were such, I would have said..."; the judge only bases his decision on what he sees: Niddah 20b
If a student hears a sage rule on a matter involving a biblical prohibition and he has a question, he should ask immediately. On a matter involving a rabbinic prohibition, he should wait until afterward and then ask simply for the sake of knowing the answer: Eruvin 67b
Weighing Opinions - General Rules
Following one school of law, and not picking from individual sides as one sees fit: Eruvin 6b-7a; Rosh HaShanah 14b
Following an individual opinion in a case of great need: Eruvin 46a
Following a sage of a previous generation, vs. a sage of the current generation, or not: Eruvin 41a
Not looking for a second opinion: Niddah 20b
Not practicing two rival stringencies [Chumrot] or leniencies [Kulot]: Rosh HaShanah 14b; Eruvin 6b-7a
Listening to a Divine Voice for Law: Eruvin 6b-7a; Pesachim 114a
Law is like an Unattributed Mishnah: Shabbat 112b, 147b, 148b, 156b; Beitzah 2a-b; Menachot 52b
Establishing that two different existent versions of a Blessing should be used in combination, rather than choose between them: Megillah 21b; Taanit 7a
The strength of a permissive authority is greater than that of a prohibiting authority (in that his opinion says more about the law, or in that he must be confident in order to permit this; depends on the context) [Koach deHetera Adif]: Eruvin 72b; Beitzah 2b Rabbi Akiva replying to Rabbi Yehoshua: If your halachic statement is a received tradition, I accept it. If it is a result of "kal vachomer" logical deduction, though, then I can refute it thus: Keritot 15b
Weighing Opinions of Specific Sages of the Mishnaic Era [Tannaim]
House of Hillel vs. House of Shammai: Eruvin 6b-7a, 13b; Pesachim 114a
The opinion of Beit Shammai is as nothing when it conflicts with that of Beit Hillel: Beitzah 11b
Law is like R' Shimon Ben Gamliel in all Mishnayot save 3, according to R' Yochanan: Bava Metzia 38b
Where you have R' Shimon vs. R' Yosi and a Mishnaic anecdote: Rosh HaShanah 22a R' Meir vs. R' Shimon: Eruvin 46b R' Meir vs. R' Yehudah: Eruvin 46b, 47a R' Yosi vs. R' Yehudah: Eruvin 46b-47b R' Yosi vs. R' Meir: Eruvin 46b-47a R' Yosi vs. R' Shimon: Eruvin 46b R' Yehudah vs. R' Shimon: Eruvin 46b
Ruling with R' Yosi, because he always has a reason for his legal decisions [Nimuko Imo]: Eruvin 14b, 51a Rebbe against an individual, possibly even against a group: Eruvin 46b; Pesachim 27a; Ketuvot 21a R' Akiva against an individual: Eruvin 46b R' Yosi against a majority: Eruvin 46b R' Yosi beRabbi Yehudah against the sages: Eruvin 16b
The law always follows R' Yehudah on issues of merging properties to permit transportation between them on Shabbat: Eruvin 81b, 95a R' Shimon on most Shabbat cases: Shabbat 157a; Beitzah 2-ab R' Yehudah on most Yom Tov cases: Beitzah 2-ab R' Elazar on most cases of Bills of Divorce: Gittin 4a R' Yochanan over Rav except in three cases: Beitzah 4b, 5b [bottom] Rav against Shemuel [in non-financial cases], except in 3 cases: Menachot 41b
R' Acha bar Chanina said, "It is revealed and known before the Creator of the Universe that there was no one in R' Meir's generation who was R' Meir's peer. The law doesn't follow him, only because his peers could not follow his logic": Eruvin 13b
Rebbe saying that R' Eliezer is worthy of being followed [against the majority] in a case of need: Eruvin 46a; Niddah 6a-b, 9b
Whether the law follows R' Meir whenever he is stringent: Eruvin 47a
R' Eliezer ben Yaakov's teachings are "Kav veNaki" - rare but refined - and the law will follow them: Eruvin 62b
Weighing Opinions of Specific Sages of the Gemara Era [Amoraim]
Considering Rav a Mishnaic sage, since he overlapped that era: Eruvin 50b; Ketuvot 8a Rav Nachman in financial issues: Ketuvot 13a Abayye in six cases: Bava Metzia 22b
Majority vs. Minority
One doesn't fulfill a Mitzvah by practicing it in accordance with a minority ruling: Berachot 10b, 11a; Tos. Berachot 11a #2
Deciding law by weighing the number of arguments for one side against those for the other: Pesachim 114a
Where an Individual stands against a Majority of Decisors: Berachot 9a; Shabbat 130b; Eruvin 46a-b; Succah 19b; Beitzah 11a; Moed Katan 20a; Ketuvot 21a; Niddah 6b, 9b Ex-communicating an individual who rules against the majority: Berachot 19a [see Rashi "medameh"]
Cases which "Break the Rules"
In cases where varying working conditions could alter the law, we may only rely on what the Sages have set down: Pesachim 48b
Extrapolating other laws from a rule that applies in a case of danger: Succah 14b
Cases in which being lenient could result in greater disruption of Jewish law, because our actions would be misinterpreted by a secular government: Beitzah 6a
Areas of Law which are Unique
Dealing differently with a case which the Torah creates as a novelty ["Chiddush"], and is therefore potentially capable of breaking ordinary rules: Pesachim 44b; Nedarim 4a; Kiddushin 21b [2x]
The law follows the lenient opinion in cases of extending the limit [Techum] for travel outside a town on Shabbat: Eruvin 46a-b, 80a
We don't follow a sage's view for law in cases of merging properties to permit transportation between them on Shabbat if the ruling incorporates two stringent positions in which he disagreed with the other sages: Eruvin 71b
Learning General Prohibitions from Rules by [Im-]purity: Yevamot 103b
Learning laws for non-sanctified items, from Sanctified items: Pesachim 45a
Discerning Standard Prohibitions from Financial Law: Berachot 19b; Kiddushin 3b; Bava Metzia 20b
Discerning between violations involving action and violations involving inaction [shev v'al taaseh]: Berachot 20a
Discerning cases involving fines, from general financial law: Kiddushin 3b
We rule leniently by laws of Mourning: Eruvin 46a-b; Moed Katan 18a, 19b, 20a, 22a
Being more lenient regarding Shabbat than Yom Tov, because Yom Tov is inherently more lenient in its laws, leading people to possibly disregard its prohibitions: Beitzah 2a-b
Being lenient regarding actions that are associated with a mitzvah that overrides Shabbat, lest one otherwise not perform the mitzvah at all [hitiru sofan mishum techilatan]: Shabbat 130a, 131a-b, 133a; Eruvin 102b-103b; Beitzah 11b
Dissenting from Standard Practice
Cases where if a person follows a given practice, it shows that "He doesn't have the spirit of the Sages": Kiddushin 17b
Contradicting a ruling because we find reality to differ from the legal perspective: Succah 8b
Punishment for acting in accordance with the House of Shammai, against the House of Hillel; or in reverse according to the House of Shammai: Berachot 11a; Eruvin 13b
Not contradicting the local authority: Eruvin 94a
Miscellaneous Principles
Avoiding causing a burden on the community [tircha d'tzibbura] in an enactment: Berachot 21a
Financial Concerns don't outweigh Concerns regarding potential Prohibitions: Bava Metzia 30a
One should not ridicule a legal opinion requiring a stringency [chumrah], even if one holds it is incorrect: Eruvin 47b-48a
"Because we imagine it [to be so], shall we then rule this way practically?!": Gittin 19a
Turning an item which was "anti" something, into a "positive" element [I know this is vague; see the case]: Kiddushin 5a
Asking why a law should be split between two, apparently similar, cases ["Mai Paska"]: Nedarim 39a [2b]
Allowing an apparent violation of a principle, simply because the alternative is impossible in practice: Megillah 27a
A student should try to think independently and creatively: Rashi Eruvin 13a "Lo Amrah"
The Torah was not given to angels ["lo nitnah Torah l'malachei hasharet"], and therefore the law must be practicable: Berachot 25b
Whether a voice from heaven [bat kol] can teach Torah, providing a legal ruling: Bava Metzia 59b
