A zakein mamre is one who violates the issur of lo tasur by giving a hora’ah/psak that conflicts with the ruling of Sanhedrin on an issue which can lead to an issur kareis. The Rambam in Hil Mamrim (1:2) writes that even if the disagreement revolves around an issur derabbanan, one can still become a zakein mamre – for example, if one paskens that chameitz during the sixth hour of the day on erev Pesach (which is only assur m’derabbanan) is permitted b’hana’ah, that chameitz might be used for kiddushin, creating an eishes ish status with issurei kareis for an adulterer all based on a psak that contradicts the Sanhedrin. Even though the law enacted by the Sanhedrin is not a Torah law, one can still be in violation of a d'oraysa of lo tasur by disagreeing with such a din.
The Ramban in his hasagot to the first shoresh in Sefer haMitzvot attacks the Rambam’s position. If indeed every derabbanan hinges on a d’oraysa obligation of lo tasur to obey the Sanhedrin, why are we more lenient with respect to dinei derabbanan – ultimately, the violation of a derabbanan is itself a violation of the d’oraysa of lo tasur? Why are we lenient by a safek derabbanan but not a safek d’oraysa when every derabbanan itself is a potential safek violation of lo tasur?
According to the Ramban, one is forced to say that dinei derabbanan are not connected with the command of lo tasur or any other specific mitzvah, otherwise one cannot avoid the questions raised by his hasaga. If so, why is one ever obligated to follow the enactments of Sanhedrin? R’ Elchanan’s suggests that one can understand the Ramban in light of his chiddush (quoted yesterday) that the ‘ratzon Hashem’ must be obeyed even without a specific command to do so. A din derabbanan by definition reflects the Sanhedrin's understanding of the ratzon Hashem, which itself, even without a specific mitzva, requires being obeyed.
It is worth noting that the Meshech Chochma in P’ Shoftim defends the Rambam by proposing the exact opposite thesis as R’ Elchanan – i.e. although the Sanhedrin is granted legislative authority through lo tasur, one cannot say that each specific law they create is the ratzon Hashem the way a mitzvah is. It is worth taking a look at the machlokes Rambam/Ramban in greater depth, but for now, more to say on this chiddush of R’ Elchanan. Stay tuned…..