Yesterday, we noted the question raised by the Magen Avraham (19:1) as to why the berakha over the mitzva of tzitzit is recited when one dons his tallit, rather than when the tzitzit are affixed to the garment, whereas the berakha over mezuza is recited when the mezuza is affixed to the doorframe. Seemingly, the berakha over mezuza should be recited when one moves into his home, just as the berakha over tzitzit is recited when one puts on the garment. The Magen Avraham answers that indeed, one who affixes the mezuzot before he moves into the home does not recite the berakha at that point, and rather waits until he moves into the home, similar to the recitation of the berakha over the mitzva of tzitzit. However, since the common custom is to affix the mezuzot only after moving into a new home, the berakha cannot be recited when one moves into the home – as the mitzva is not yet being fulfilled – and it is thus recited when the mezuzot are affixed.
Rav Asher Weiss, however, suggests other reasons for this distinction between the berakha over tzitzit and the berakha over mezuza. Firstly, several sources indicate that berakhot over mitzvot are recited only before performing a “ma’aseh mitzva” – a concrete mitzva act. If the mitzva’s fulfillment does not coincide with the concrete action which it requires, then the berakha is recited before performing the action, even though the mitzva is not fulfilled at that moment. And in a situation where the mitzva’s fulfillment does not require any concrete action, no berakha is recited. Thus, for example, as Rav Weiss notes, Rabbi Akiva Eiger rules (responsa, 9) that one does not recite a berakha over the mitzva of mezuza if he moves into a house that already has mezuzot, or if he moves back into his home after spending an extended period away from home. Rabbi Akiva Eiger writes that in these two cases, one fulfills the mitzva of mezuza by moving into, or back into, the home, but he nevertheless does not recite the berakha because he does not perform a concrete mitzva act.
For the same reason, Rav Weiss explains, Chazal instituted the recitation of a berakha when affixing the mezuza, and not when one moves into the home. Since affixing the mezuza constitutes the “ma’aseh mitzva” – the mitzva act – it is then when the berakha must be recited. Moving into a home does not qualify as a mitzva act, and thus the berakha cannot be recited at that point. And herein, Rav Weiss writes, lies the difference between mezuza and tzitzit. The act of putting on a tallit constitutes a mitzva act, and thus the berakha over the mitzva of tzitzit was instituted to be recited at that point. When it comes to moving into a home, by contrast, although the mitzva of mezuza is, technically speaking, fulfilled at that point, the mitzva act is the affixing of the mezuza to the doorframe, and it is thus on that occasion when the berakha is recited.
Rav Weiss then suggests a more fundamental distinction between the two mitzvot, asserting that while they may appear similar, the respective definitions of these obligations in truth differ significantly from one another. When it comes to tzitzit, the mitzva is defined as an obligation to wear tzitzit. Wearing the tzitzit constitutes the essential definition of the mitzva, whereas the need for a four-cornered garment is a detail that limits its scope. The mitzva of mezuza, by contrast, is defined not as an obligation to live in a home with a mezuza, but rather to ensure that the home in which one resides has a mezuza. Here, the essential definition of the mitzva is that the home should have a mezuza, and the person’s ownership of the home is a peripheral detail. And for this reason, the berakha over tzitzit is recited when we put on the tallit, whereas the berakha over mezuza is recited when the mezuza is affixed to the doorframe. The mitzva of tzitzit is fulfilled at the time one puts on the garment, whereas the mitzva of mezuza is fulfilled when the mezuza is affixed to the door, and this accounts for the difference in the recitation of the berakhot.