R’ Aharon Rakeffet has a 10 year series on responsa literature and the art of making a halachic rulings. The classes are available on YUTorah.org. The following is primarily from his shiur of Dec. 19th, 1994 “Safek from Torah or Rabbanan” (starting at around 52 min. in). As is my norm, I add bits here and there.
The Rama writes that if one can not find a reason to choose one side of a machloqes (dispute) over another, he must use the rules of doubt. Which means that if the halakhah is fiscal, then the person holding the money keeps the money (hamotzi meichaveiro alav hara’ayah – he who wants to take from his peer has the burden of proof, “possession is 9/10 of the law”); when it is Torah prohibition or obligation, one must rule strictly; and in rabbinic prohibitions or obligations, one is supposed to be lenient. And thus much of responsa literature is about figuring out whether the prohibition is Torahitic or rabbinic. And once you find out it’s derabbanan, you can start adding up senifim lehaqeil, flaws in taking the law for granted, until one can consider the cumulative doubt sufficient to say safeiq deRabbanan lehaqeil.
But why do we rule leniently for a rabbinic law? Isn’t every rabbinic law really a Torah law of “do not veer from what they tell you, neither to the left nor to the right”?
1- Ramban (on Seifer haMizvos, shoresh 1): The same Rabbis who made the rabbinic prohibitions and duties made them only applicable in the case of certainty. They desired to make a clear distinction between Torah and rabbinic law.
2- [My own addition] R’ Shimon Shkop allows us to say the same thing, 180º off. In Shaarei Yosher, he asks the question of why a sefeiq sefeiqah (a doubt added upon a second doubt) is ruled leniently.
The Rashba (Shu”t 1:401) holds it’s a variant on the notion of relying on majority. If the first doubt is roughly 50:50, and the second doubt is roughly 50:50, the chance of violation is 1/4, and therefore ignorable. However, Rav Shim’on asks, why then do we have the rule “mi’ut bemaqom safeiq lo amrinan — we do not speak of a minority added to a safeiq“? After all, if the first doubt is 50:50, any minority to whittle away at one side would make a majority? And yet, a case of doubt plus minority is no more lenient than without the minority?
Rav Shimon explains sefeiq sefeiqa on other grounds, following the Rambam. Who said that a doubt in Torah law must be ruled stringently? It wasn’t the Torah, it is rabbinic! And therefore, a second doubt on top of that first one is a doubt in a rabbinic law — and therefore we rule leniently.
(This reasoning also argues for accepting a sefeiq sefeiqa she’eina mis-hapekhes (an issue at the core of eating chadash), but that discussion would take us even further afield.)
So, rather than the Ramban’s limiting the specific prohibition to only cases where we are certain about the realia, it’s possible that we could limit the rabbinic enactment of ruling stringently on Torah law to have only been made about the other 612 laws. With the same consequent rationale.
3- Rav Meir Simchah haKohein miDvinsk (Meshekh Chokhmah, Devarim 17:11): A Torahitic prohibition describes something that is inherently wrong. The universe is made such that combining meat and milk is a problem (metu’af, meshuqatz).
A rabbinic prohibition lacks that reality. Chicken and milk isn’t inherently damaging, it is that it leads to error through habit or accident. Therefore, one needn’t the same care when dealing with rabbinic extension as when dealing with the damaging or refining thing itself.
4- Rav Elchanan Wasserman (Qunterus Divrei Soferim): Of course there is a reality to rabbinic statements. It is all revealed from the Creator, all the Ratzon Hashem yisbarakh (the Will of the Creator, blessed be He). The difference between a derabbanan and a de’oraisa is the explicitness. Therefore it is less sacred, and violation involves lesser realities. A difference of quantity, not quality.
Rabbi Rakeffet links Rav Elchanan’s position to his belief in da’as Torah; both imply a belief that there is revelation of Hashem’s Will today through the rabbis.
5- Shulchan Arukh haRav [another addition not in the lecture]: In a rare case of where the Shulchan Arukh haRav discusses the purpose of a law rather than just codifying practice, he discusses the significance of yom tov sheini shel galiyos, the observance of a second day of Yom Tov outside of Israel.
He explains that there is no time in the heavenly realms. The supernal “Pesach” is not associated with any particular time. Hashem made a connection between that Pesach and the 15th of Nissan, giving us a worldly manifestation within time. The SAhR continues that the 16th of Nissan is connected to the very same supernal Pesach. The seder on the 2nd night is a manifestation of the same metaphysical reality. What differs is who draws down the connection, not what it is we are connected to.
Perhaps this is generalizable to rabbinic legislation in general. This would result in an opinion similar to Rav Elchanan’s in that it gives a reality to rabbinic law, rather than their just being pragmatics for how to keep Torah law. However, the opinions are also quite different in that it makes the rabbinic legislator a metaphysical engineer, building the reality, rather than a conduit of Hashem’s revelation of that reality.
And, to continue R’ Rakeffet’s thought, Chassidic attachment to the Tzaddiq is not the same as the Yeshiva World’s notion da’as Torah.
6- Seifer Me’iras Einayim (SM”A, Ch”M 67, #2): The berakhah that Hashem gives to those who keep shemittah , that they will have sufficient crops in the 6th year for the 6th, 7th and 8th years, is only when shemittah is mandatory by Torah law. (I.e. when the majority of the tribes are in their lands, and therefore there is a yoveil every 50th year.) Today, someone who keeps rabbinic shemittah gets no such guarantees.
7- Chazon Ish (Deshevi’is 18, #4): The blessing did apply during the 2nd Temple and after its destruction, for the heavenly court fulfills based on what’s decreed down below.
Rabbi Rakeffet identifies the SMA with the position of the Meshekh Chokhmah, and the Chazon Ish with R’ Elchanan Wassermnn’s. To my mind, it’s possible that his position is more like the Shulchan Arukh haRav.
However, this explains why the Chazon Ish was so willing to be stringent when it came to keeping shemittah. Had he felt that the observance didn’t come with insurance from the A-lmighty, perhaps he would have ruled leniently.