When I grew up, went to yeshiva and for years after I always had this picture of Torah Sheba’al Peh – the oral law – as all encompassing. It covered everything from the early Mishna until the Halachik ruling of a current great Posek. Disagreeing with any of this large corpus was unthinkable. It created problems internally because it did not make sense that there is a body of such magnitude that is inviolate and sacred especially when many rules were irrational. I alternated between feeling guilty and stupid. I then realized that I was operating under a misconception and I should have felt guilty for being so ignorant and not for being rational. In this post, I will focus on a very limited aspect of the whole issue, trying to define the immutable part of the oral law, as I understand Rambam’s position.
In his introduction to the Pirush haMishna Rambam tells us that Moshe received the written law verbatim from God accompanied with detailed explanations of how each Mitzvah had to be implemented.
דע, כי כל מצווה שנתן הקב"ה למשה רבנו ע"ה, נתנה לו בפירושה: היה אומר לו
המצווה, ואחר כך אומר לו פירושה ועניינה, וכל מה שהוא כולל ספר התורה.
הנה לך משל: שהקב"ה אמר למשה, בסכות תשבו שבעת ימים (ויקרא כג). אחר כן
הודיע שהסוכה הזאת חובה על הזכרים לא על הנקבות, ושאין החולים חייבים בה
ולא הולכי דרך, ושלא יהיה סיכוכה אלא בצמח הארץ, ולא יסככנה בצמר ולא במשי
ולא בכלים, אפילו מאשר תצמח הארץ, כגון הכסתות והכרים והבגדים. והודיע
שהאכילה והשתייה והשינה בה כולו חובה, ושלא יהיה בחללה פחות משבעה טפחים
אורך על שבעה טפחים רוחב, ושלא יהיה גובה הסוכה פחות מעשרה טפחים.
וכן השש מאות ושלש עשרה מצות הם ופירושם: המצוות בכתב, והפירוש על פה.
For example, when God said to Moshe that you should sit in a Sukkah seven days he also told him that only males and not females are obligated, sick and travelers are exempt. He also told him that the covering should be only from plants of the earth, not from wool or silk. The covering could also not be made up of finished products even if they were made of plant, like pillows, covers or clothing. He was also told that eating and sleeping in the Sukkah is obligatory, that it has to have a space of seven by seven Tefachim and that it cannot be higher than twenty Amot.
So it went with all the 613 Mitzvot – the Mitzvah in writing and the explanation orally.
These explanations were memorized and transmitted from generation to generation as long as there was a gathering of scholars. We find these oral transmissions embedded in the Gemara and the Halacha. The last reliable source for this transmitted law is the Beit Din that sealed the Talmud (See Introduction to MT). Rambam emphatically states that there never was any argument about these received and carefully transmitted explanations. (Discussion of this particular point covers many pages of commentators, critics and defenders).
Along with these explanations came a list of 13 hermeneutical rules to be used for extrapolating new applications of the basic law. During Moshe’s lifetime, the Sanhedrin convened almost constantly and new applications of the law were developed to address the circumstances as they arose. There were so many developed over the forty years that when Moshe died a small portion, three thousand, of these developed applications were forgotten. That gives us an idea of how many were developed during Moshe’s life - probably tens of thousands.
These new applications of the law were decided by the Sanhedrin using the hermeneutical rules. If there was a disagreement, the vote of the majority became law. These laws were not eternal and could be overturned by subsequent Sanhedrin if the majority of their era ruled differently.
The picture that emerges is that the oral law that is Sinaitic and immutable is limited to the ones that lay out the basic rule of how a Mitzvah is practiced. Any expansion of that explanation falls under a different rubric – a law established by Sanhedrin. That may change from Sanhedrin to Sanhedrin. The immutable corpus is thus quite limited. (Caution: I over simplified for coherence sake. There are categories within the corpus of laws developed by Sanhedrin that have different rules such as Gezeirot and Minhagim. Rules of precedent law differ in each category. I will leave this for another discussion.)
However, as times got bad and the Jewish people dispersed farther around the globe, Rebbi in his Mishna followed by Rav Ashi with the Talmud, collected and analyzed the rulings of the Sanhedrin and central Batei Din up to their time. These laws, and only these laws, were now sealed and unchangeable until Sanhedrin return to us. Our law is built on that foundation. New applications and rulings that arise to deal with newly developing circumstances after the sealing of the Talmud, are only binding on each locality. There is no more universal law. 
The goal of Rambam in his Mishne Torah was to gather up in a systematic and organized format all the rulings found in the Mishna, Talmud and the corpus of sources that support them. Those were written in an argumentative style which makes it quite complicated to arrive at clear conclusions. Rambam undertook to present the conclusions in a clear and succinct presentation. These are the rulings that cannot be changed and are binding until Sanhedrin returns. (Of course, even here not everybody accepted Rambam’s conclusions. But that is so in a relatively few number of instances. This discussion is also not for here.)
In subsequent generations, codification of the subsequent rulings after the Talmud was attempted. The most famous ones were the Tur, the Shulchan Aruch and up to our times, this process continues with the Aruch Hashulchan and the Mishna Berurah. These are all-important works and are accepted in general by our communities. However, they are not inviolate and as anyone who learns knows, local custom and rulings supersedes these codes. They are more a collection of reliable rulings than conclusive codifications.
The point of this post is to respond to many comments and sometimes outraged ones, reacting to statements I made about laws being binding while at the same time suggesting that future Sanhedrin might change things. Because of the current state of affairs of the Jewish people, the discord, the attitude towards any “other”, paranoia about “kefirah” and other such aberrations, there is no possibility of a consensus for a Sanhedrin. We are therefore hobbled by our own fault, and many incongruous and unfortunate situations, some more painful than others, cannot be rectified. We desperately need some forceful and visionary leadership with a radical change in the general attitude.
כל עיר ועיר וכל מדינה ומדינה לנהוג בכל המנהגות שנהגו חכמים שבתלמוד,
ולגזור גזירותם וללכת בתקנותם.
לה הואיל וכל אותן הדברים שבתלמוד הסכימו עליהם כל ישראל,
ואותן החכמים שהתקינו או שגזרו או שהנהיגו או שדנו דין ולמדו שהמשפט כך
הוא הם כל חכמי ישראל או רובן, והם ששמעו הקבלה בעיקרי התורה כולה, איש
מפי איש עד משה רבנו.
 The Yeshivot in Bavel were considered as central Batei Din with authority similar to Sanhedrin in many areas.
 וכל בית דין שעמד אחר התלמוד בכל מדינה ומדינה וגזר או התקין או הנהיג
לבני מדינתו, או לבני מדינות--לא פשטו מעשיו בכל ישראל: מפני רוחק
מושבותיהם, ושיבוש הדרכים; והיות בית דין של אותה המדינה יחידים, ובית דין
הגדול של שבעים בטל מכמה שנים קודם חיבור התלמוד.
ואין אומרין לבית דין זה לגזור גזירה שגזרה בית דין אחר במדינתו. וכן אם
למד אחד מן הגאונים שדרך המשפט כך הוא, ונתבאר לבית דין אחר שעמד אחריו
שאין זה דרך המשפט הכתוב בתלמוד--אין שומעין לראשון, אלא למי שהדעת נוטה
לדבריו, בין ראשון, בין אחרון.