lo tasur and dinim derabbanan

Returning to R’ Elchanan’s Kuntres Divrei Sofrim, I am going to do some jumping rather than slavishly follow the order of the sefer point by point. Continuing with the topic of zakein mamrei, the Rambam writes that one can become a zakein mamrei by challenging Bais Din even on a din derabbanan provided it is a detail of an issur which is categorized among those which can lead to a punishment of kareis. The Ramban disagrees. The prohibition of zakein mamrei stems from the pasuk of “lo tasur”, which (Ramban writes) could not possibly refer to dinim derabbanan. If every derabbanan was included in the prohibition of “lo tasur”, why should there be any difference between dinim d’oraysa and dinim derabbanan? Why is it that s’feika d’oraysa l’chumra, but sfeika derabbanan l’kula? One could perhaps say that the rabbanan legislated that one need not be stringent in cases of doubt, but that approach seems forced and only begs the question of why there was a need to make any distinction.

Implicit in the Ramban’s attack is a rejection of “lo tasur” as the basis for obeying Rabbinic law. But if the need to obey a din derabbanan is not rooted in some pasuk, why indeed are we bound to obey? R’ Elchanan suggests a chiddush: a Jew is bound to obey the will of G-d. That will can be expressed in the form of a Biblical command, but it also might remain for us to intuit without an explicit command. The basis for all Rabbinic law is our trust that the Rabbis successfully intuited the will of G-d and revealed it to us.

I already did some posts in the past on this chiddush (here, here, here, and here) and will just bli neder just briefly review some of REW’s proofs. One point to ponder which R' Elchanan comes back to: granted that according to Ramban there is no pasuk in which dinim derabbanan are rooted, but given that dinim derabbanan are as much the will of G-d as a Biblical command, why (as the Ramban himself asks!) do they not carry the same stringency?