As we noted yesterday, the Rambam lists in his Sefer Ha-mitzvot (asei 34) the command that the ark be transported on the carriers� shoulders, and not by wagon, based on a verse in Parashat Naso (�ba-kateif yisa�u� � 7:9).  Interestingly, the Rambam asserts that although here in Sefer Bamidbar the role of transporting the ark was assigned to the entire Kehat family of Leviyim, the mitzva is fundamentally cast specifically upon the kohanim.  During the period of Benei Yisrael�s travels in the wilderness, the Rambam explains, there simply were too few kohanim to carry the heavy ark, and therefore God assigned this task to the entire family of Kehat (which included the kohanim; Kehat was Aharon�s grandfather).  Later, however, as the population of kohanim grew, this role became the exclusive province of the kohanim.  The Rambam notes a number of instances in the Nevi�im where it was specifically the kohanim who transported the aron. 

            The Ramban, in his critique of Sefer Ha-mitzvot, rejects the Rambam�s contention, citing several proofs to the fact that even after the period of the wilderness this role was performed by Leviyim, and not kohanim.  He begins his argument by claiming, �Heaven forefend [chalila lanu] that we should say that any mitzva of the Torah changes, such that the Leviyim should ever become disqualified from carrying the ark.�  The Ramban here advanced an ideological challenge to the Rambam�s view, objecting to the possibility of a mitzva changing from one generation to the next. 

            Rav Asher Weiss, in his Minchat Asher (Parashat Naso, siman 14), suggests defending the Rambam�s stance in light of his formulation in defining this mitzva in his Sefer Ha-mitzvot Ha-katzar (brief listing of the mitzvot at the beginning of Mishneh Torah).  There the Rambam writes, �to carry the ark on the shoulder when it is transported.�  Revealingly, the Rambam does not many any mention here of the people assigned this task.  He defines the mitzva as requiring transporting the ark by shoulder, without specifying that this obligation is assigned to the kohanim.  This is in contrast to other mitzvot that are assigned to the kohanim or the Leviyim, in the context of which the Rambam in Sefer Ha-mitzvot Ha-katzar specifies upon whom the obligation rests.  For example, the 24^th mitzva is �the kohen washes his hands and feet� before serving in the Temple.  Rav Weiss further notes that even in Sefer Ha-mitzvot, where the Rambam does mention the specific group charged with this obligation, his formulation is unusual: �He commanded us that the kohanim should carry the aron on their shoulders.�  In defining other mitzvot assigned to the kohanim, the Rambam writes, �The kohanim are commanded,� whereas here he writes that �we� are commanded that the kohanim should transport the ark upon their shoulders. 

            These nuances, Rav Weiss suggests, might indicate that the Rambam did not view this mitzva as fundamentally cast upon any particular group.  As opposed to the rituals in the Mikdash, for example, which are inherently exclusive to the kohanim, the job of transporting the ark is not intrinsically associated with one group or another.  The essential definition of the mitzva is that it must be transported by shoulder; the question of who does the transporting is just a detail.   

            And this might perhaps be the Rambam�s response to the Ramban�s objection.  The Ramban is correct, that a mitzva of the Torah will not change from one generation to the next.  However, this is true only of the essential definition of the mitzva.  In this instance, the mitzva is fundamentally defined as transporting the ark by shoulder, which indeed applies eternally.  The question of who transports the ark is a secondary detail, which may, indeed, depend on the time and circumstances.