At the conclusion of this week’s parshah Hashem commands Bnei Yisrael to wear tefillin. The Torah says that we should watch this mitzvah “from days to days.” The explains that these words refer to the obligation to check one’s tefillin every year.
The Meshech Chachmah quotes the Mishkenos Yaakov (Yoreh Deah 15) who derives from this Mechilta that one must check something even if a rov or chazakah attest to its validity. In other words, although most tefillin are kosher one year after being checked, the Torah obligates us to check them nonetheless.
The Mishkenos Yaakov asks: If so, why is there only a rabbinical obligation to check the lungs of an animal. It’s true that the majority of animal lungs are not tereifos, but don’t we learn from the Mechilta that, biblically, we are not supposed to rely on a rov if the matter can be checked into?
(As a side note, the reason why the Mishkenos Yaakov based his question on the halacha of checking for tereifos in lungs and not on the halacha requiring us to check for other tereifos is because other ones are extremely rare. Indeed, one is not even obligated to check for them m’d’Rabanan. Tereifos in the lungs, though, are considered a mei’ut hamatzoi – frequent minority. Other teraifos are considered mei’uta d’mei’uta, which the Mishkenos Yaakov defines as being in less than 10 percent.)
The Meshech Chachmah answer the Mishkenos Yaakov’s question by differentiating between mitzvos and aveiros. The Mechilta mentioned above requires one to check one’s tefillin once a year. This obligation ensures that the performance of a mitzvah is adhered to. If we were to follow the rov and assume that the tefillin are kosher, one may wind up not fulfilling a mitzvah – since the tefillin may later be found to be pasul. In order to ensure that the mitzvah is fulfilled the Mechilta obligated us to check our tefillin annually.
The worse that will result in relying on the rov in regards to tereifos, however, is an aveirah b’ones – an unwilling aveirah. Therefore, while there an obligation to check for mitzvos, there is no similar obligation min HaTorah for aveiros if there is a rov.
This answer – which differentiates between mitzvos and aveiros – is used to explain the Rambam’s opinion regarding sefeikos d’Oraisa. The Rambam opines that a safek d’Oraisa is only to be treated l’chumra m’d’Rabanan. M’d’Oraisa one can be lenient. Rishonim are perplexed by this opinion based on a passage in the Gemara (Chullin 11a). This passage seeks to find a source from the Torah that we can rely on a rov. It cites as proof the halacha that we don’t check the korban Pesach for tereifos because we fear that we may break a bone, which is assur. How then can we eat the korban Pesach? Evidently the Torah intended for us to rely on the rov of animals and assume it is not a tereifah.
But according to the Rambam, what proof is this? Perhaps the reason the animal is permitted is because safek d’Oraisa l’kulah, not because we rely on a rov?
Acharomin (Chavas Daas, Rav Akiva Eiger) answer that the Rambam agrees that safek d’Oraisa l’kulah isn’t enough to when it comes to fulfilling mitzvos. If one is unsure, one must fulfill the mitzvah again. The fact that one may go ahead and eat the korban Pesach without checking, therefore, must be because of a difference factor – the rov.
In conclusion, one may biblically rely on the rov for aveiros but not for mitzvos when double checking is a possibility.