Klalei HaPoskim

Overview

In order to be privy to the nuances and mindsets of the Poskim, it is advantageous to understand the Klalim relevant to the Poskim, from the two Talmuds through the Rishonim to the Shulchan Aruch. The Machon Yerushalayim edition of the Yad Malachi is particularly helpful for this, as the Yad Malachi is fantastic for collecting all the ideas mentioned throughout Halachic literature and the footnotes add many additional sources from those who came before and after him, as well as direct quotations of sources.

Geonim

1. The term "Geonim" generally refers to those who lived between the Savoraim and the Rif, though the institution of Geonim did extend beyond then. Some say that the term Gaon is short for "Rosh Yeshivat Gaon Yaakov," while others claim it was reserved for those who were experts in all sixty Masechtot of Shas, corresponding to the Gematria of גאון.
2. The monograph on Hilchot Tefillin from the times of the Geonim is claimed by some to be super authoritative, as "the words of the Geonim are tradition" (דבריהם דברי קבלה) in that they had access to the first edition of Talmud Bavli, which was clearer than ours. At the same time, a number of Rishonim poke holes in the work and argue it shouldn't be followed when it contradicts Talmud Bavli.

Bahag (Ba'al Halachot Gedolot)

1. Often times, the Rishonim will disagree with the Bahag but conclude in favor of his ruling anyway, even if it's illogical, because "Devarav Divrei Kabbalah Hem." Because of the tradition stemming from the Amoraim, through the Savoraim, to him, the Bahag is granted special authority when the Gemara does not explicitly say otherwise; however, if the Rishonim feel the Gemara contradicts his words, then they freely disagree in practice, as well.
2. The Rishonim are uncertain if the Halachot Gedolot was authored by Rav Yehudai Gaon, who was blind, or by Rav Shimon Kayyara. Illogical rulings could be pinned on the former's students misinterpretation of their rebbe's words when writing his sefer. Some say Rav Shimon Kayyara wrote Halachot Gedolot, and Rav Yehudai Gaon wrote a different sefer known as Halachot Pesukot, while others claim they each wrote a sefer called Halachot Gedolot.

Rif

1. Similar to the rule of Stam vaYesh in reading Shulchan Aruch, when the Rif states that the Halacha follows a certain ruling but there are those who disagree, he intends to rule like the former and is only sharing the latter to transmit the rulings of earlier generations.
2. Even though the Rif generally only records Halachot that are relevant nowadays, for example, to the exclusion of Kodashim, if a Halacha that is generally relevant has a detail that is not, he will write the entire Halacha including the detail. Moreover, if there is some relevant Halacha or Chiddush to derive from the irrelevant Halacha, then that Halacha will be included, as well.
3. Unless explicitly stated by the Rif that one should be stringent, the assumption is that a lenient approach is taken with unresolved inquiries of the Gemara (Tiku)

Rambam

General

1. The Maharashdam believes that since the Rambam saw the other opinions on each issue and still decided the way we did, we should follow his rulings (analogous to the Radbaz and the Rif). On the other hand, a number of Ashkenazi authorities feel that the Rambam is a minority in the face of the Baalei HaTosafot.
2. One cannot pose questions from Tosafot's logic against the Rambam. Gavra aGavra KaRamit?!
3. The Rivash and Rosh warn that one who attempts to rule on practical matters solely from Mishneh Torah without understanding its Talmudic context will likely mistake Assur for Muttar and Muttar for Assur, thinking he understands the material. At the same time, the Rambam's explicit intention was to write the Halacha so clearly that one need not be bogged down by the confusing Sugya and need only Mishneh Torah to know how to act, so some Acharonim, such as the Ohr HaChaim and Baal HaTanya are less concerned. While that is true, the Rambam himself explains in a letter that he did so for people for whom understanding the Gemara was beyond their reach.
4. Seldom does the Rambam disagree with the Rif, so one should not assume so unless there are already a great number of Rishonim for him to side with.

Mishneh Torah

1. In Mishneh Torah, the Rambam focuses on relaying rulings explicated in the Gemara in a lucid way, while leaving out rulings that can only be inferred but aren't stated explicitly.
2. Every word of the Rambam is written with tremendous precision and exactness, enough for one to infer from his words as one would from the Gemara itself. Similarly, the Maharalbach warns that one shouldn't rush to reject the Rambam for coming off as against the Gemara, but should rather pin the in-congruence on our own lack of understanding.
3. Though the Rambam never relies in Mishneh Torah on what he already wrote in Peirush HaMishnayot, he does rely on what he wrote in earlier sections of Mishneh Torah, and perhaps even later ones, or, at least, in that chapter.
4. When a contradiction is found between the Perush HaMishnayot and the Mishneh Torah, the Halacha follows Mishneh Torah, which was written later and as a set of rulings, not a set of elucidations of the Mishnah.
5. In Mishneh Torah, the Rambam's style is to write only what it says in the Gemara and not the interpretation, but his intention is to for the interpretation of the Gemara to be applied to his words, as well, though some disagree.
6. It's abnormal for the Rambam to omit a din dealt with explicitly in the Talmud and instead write a different idea not mentioned in the Talmud that implies the one mentioned explicitly.
7. Counts of the number of Perakim, Halachot, Mitzvot, etc are provided in Mishneh Torah to facilitate memorization, not to imply additional ideas.
8. The Rambam will rule like a Halacha found in the Sifrei if it's not contradicted by the Talmud.
9. The word Assur is used even when referring to Dinim MiDeoraita that would warrant Malkot.
10. "MiPi HaShmua" refers to a Halacha whose source is not explicit in the Torah, and "MiPi HaKabbalah" refers to Halachot learned from Pesukim in Neviim.
11. The term "MiDivrei Sofrim" can refer to Dinim MiDeRabbanan, as well as any Din MiDeoraita not stated explicitly in the Torah, such as one derived via the thirteen Middot, because without the Chachamim it would not be understood.
12. Consistently marking each Din as a Gezeirah or Halacha LeMoshe MiSinai is not a priority for the Rambam: sometime he mentions the Din's classification, while other times he does not.
13. The Rivash argues that the Rambam will refer to a Takkanat Chachamim as "Torah," such as in "Lo Hikpida Torah" in reference to the Ne'emanut of an Ed Echad, while others disagree.
14. When the Gemara presents more specific arguments predicated on accepting one of two possible assumptions - "Im Timtzi Lomar" - the Geonim and Rambam understand that the Halacha assumes like that assumption. Some say this is only true if the Gemara did not leave the very same uncertainty pending with a "Tiku" elsewhere, while others argue the opposite. The two levels of the Im Timtzi Lomar cannot be interchangeable, meaning if assuming A over B and then asking C over D is equivalent to assuming C over D and then asking A over B, this rule does not apply for the Rambam. The words must also be explicit in the text of the Gemara, so two subsequent questions, one building on the previous, would not be subject to this rule. The Poskim dispute if this is only true when the Amora in the Gemara himself uses these words to navigate the situation or if it's even true when the omniscient narrator of the Sugya does so externally of the Amora's words.
15. If the Gemara presents two opposite understandings and then rejects one, the Rambam will formulate it in a way that accepts the accepted one and implies the rejection of the rejected one.
16. Often times, answers presented in the Gemara in rejection of a difficulty are omitted by the Rambam in favor of the simple meaning of the Mishnah, Baraita, or Amoraic statement.
17. It is not uncommon for the Rambam to pin a ruling on a Pasuk not mentioned in or even rejected by the Gemara, because he felt it to be simpler or more acceptable, especially if there's no practical difference.
18. In a few places, the Rambam employs a Talmudic phrase to mean something different from what it does in the Talmud.
19. One cannot infer from the presentation of a ruling in one section of Mishneh Torah and its absence in another seemingly appropriate section that it only applies in one and not the other, as the Rambam's goal is to present the Talmud's rulings in their wording in their appropriate place. For example, a Halacha in Terumot that might also fit in Maachalot Assurot cannot be assumed to only apply in Terumot and not Maachalot Assurot given its presence in the former and absence in the latter. This is, however, a valid tool in reading other Poskim, such as the Tur.

Sefer HaMitzvot

1. The focus of Sefer HaMitzvot is not to determine which Mitzvot are part of the 613, not to present a comprehensive of accurate representation of their Halachot and details. Therefore, when studying Sefer HaMitzvot, questioning the count of Mitzvot is wholly acceptable, but questioning the details of the Mitzvot is not. The Mishneh Torah was written to address those details, and, even if there are distinct differences, it's always possible that he changed his mind over time.
2. There's a debate regarding if specific Rishonim such as the Samag, Maggid Mishneh, and Migdal Oz saw the Sefer HaMitzvot, as it wasn't translated from Arabic to Hebrew for some time. Later Acharonim culled lists of instances in which each of them cites the Sefer HaMitzvot to dispel those claims.

Perush HaMishnah

1. In Perush HaMishnayot, wherever there is an opinion that misleadingly seems to be the minority one but is, in fact, the one the Halacha follows, the Rambam will buttress that opinion by writing of its truth or singularity or the like to indicate that the Halacha does indeed follow it.
2. Sometimes our translation of Perush HaMishnayot will refer to a din as being MiDeRabbanan, but it's probably just a mistaken translation of "MiDivrei Sofrim."
3. The Ramban did not have the Introduction to Perush HaMishnayot available to him, as Hebrew translations of the Arabic were not yet available in Spain, according to the Yad Malachi, who posits that had they been available, the Ramban would not have been as quick to argue.

Ra'avad

1. In a dispute between the Rambam and Ra'avad, some argue the Halacha should be treated as a Safek, while others favor the Ra'avad when he is more stringent than the Rambam, and still others are willing to be lenient like the Ra'avad against the Rambam.
2. When the Ra'avad presents a dissenting opinion with the opening of "Yesh Mi SheOmer," the Tur seems to think he actually paskens that way, but the Beit Yosef does not.
3. In his inimitable style, the Ra'avad's critiques of the Rambam should not be seen as personal attacks to belittle the Rambam but rather as means of raising the red flag to warn people not to follow the rulings he felt the Rambam was mistaken in presenting.
4. Neither the Rambam's Perush HaMishnayot nor Moreh Nevuchim were present before the Ra'avad, as they had not yet been translated from Arabic to Hebrew.
5. "Gedolei HaMefarshim" and "Gedolei HaMagihim" refer to the Ra'avad.

Sefer Mitzvot Gadol (Semag / Samag)

1. When quoting the Gemara, the Samag will often paraphrase to make the language flow more quickly.
2. The Samag draws heavily from the Rambam for the most part and entirely from his teacher, the Sefer HaTerumah, whose Maftechot he copied letter for letter. When in agreement with the Rambam, he writes so implicitly, and he only disagrees when out-rightly quoting the Rambam by name.
3. At the same time, most of his words are based on the Tosafot Shanz, and, for some reason, he does not seem to have seen the Tosafot Tuch.
4. The Bach says the Samag's way is to rule like the Ri Baal HaTosafot.
5. There's a debate if the Samag saw the Rambam's Sefer HaMitzvot or parts of it.

Further Reading

• Yad Malachi, especially the Machon Yerushalayim annotated edition
• Shem HaGedolim
• Ein Yitzchak, by Rav Yitzchak Yosef