הָכָא טְרִיד טִירְדָּא דְמִצְוָה, הָתָם טְרִיד טִירְדָּא דִרְשׁוּת. וְהָעוֹסֵק בְּמִצְוָה פָּטוּר מִן הַמִּצְוָה מֵהָכָא נָפְקָא? מֵהָתָם נָפְקָא, דְּתַנְיָא: ״וַיְהִי אֲנָשִׁים אֲשֶׁר הָיוּ טְמֵאִים לְנֶפֶשׁ אָדָם וְגוֹ׳״ — אוֹתָם אֲנָשִׁים מִי הָיוּ? נוֹשְׂאֵי אֲרוֹנוֹ שֶׁל יוֹסֵף הָיוּ, דִּבְרֵי רַבִּי יוֹסֵי הַגְּלִילִי. רַבִּי עֲקִיבָא אוֹמֵר: מִישָׁאֵל וְאֶלְצָפָן הָיוּ, שֶׁהָיוּ עוֹסְקִין בְּנָדָב וַאֲבִיהוּא. רַבִּי יִצְחָק אוֹמֵר: אִם נוֹשְׂאֵי אֲרוֹנוֹ שֶׁל יוֹסֵף הָיוּ, כְּבָר הָיוּ יְכוֹלִין לִיטָּהֵר. אִם מִישָׁאֵל וְאֶלְצָפָן הָיוּ, יְכוֹלִין הָיוּ לִיטָּהֵר. אֶלָּא, עוֹסְקִין בְּמֵת מִצְוָה הָיוּ שֶׁחָל שְׁבִיעִי שֶׁלָּהֶן לִהְיוֹת בְּעֶרֶב פֶּסַח, שֶׁנֶּאֱמַר: ״וְלֹא יָכְלוּ לַעֲשׂוֹת הַפֶּסַח בַּיּוֹם הַהוּא״, בְּיוֹם הַהוּא אֵין יְכוֹלִין לַעֲשׂוֹת, הָא לְמָחָר — יְכוֹלִין לַעֲשׂוֹת!
The Gemara answers: Nevertheless, there is a distinction between the cases. Here, in the case of a groom, he is preoccupied with the preoccupation of a mitzva that he must perform; there, in the case of a ship lost at sea, he is preoccupied with the preoccupation of a voluntary act that he chooses to perform. § The Gemara asks: And is the halakhic principle that one who is engaged in a mitzva is exempt from performing another mitzva derived from here? It is derived from there, as it is taught in a baraita that it is written: “There were certain men who were impure by the corpse of a person and they could not observe the Pesaḥ on that day” (Numbers 9:6). Before proceeding with the discussion, the baraita seeks to clarify with regard to those men who became impure: Who were they? The baraita answers: They were the bearers of Joseph’s coffin, which the Jewish people brought with them in the desert. This is the statement of Rabbi Yosei HaGelili. Rabbi Akiva says: They were Mishael and Elzaphan, who were engaged in carrying the bodies of Nadav and Avihu after they were burned in the Holy of Holies (see Leviticus 10:4). Rabbi Yitzḥak says: These identifications are inaccurate, because if they were the bearers of Joseph’s coffin, they could have already been purified. They were camped at Sinai sufficient time to become purified in time to sacrifice the Paschal lamb. And if they were Mishael and Elzaphan they could have already been purified, as the Tabernacle was erected on the first of Nisan, which was the eighth day of the inauguration, when the sons of Aaron were burned. More than seven days remained until the eve of Passover on the fourteenth of Nisan. Rather, they were unnamed people who were engaged in tending to a corpse whose burial is a mitzva, i.e., which has no one else available to bury it, and their seventh day of impurity occurred precisely on the eve of Passover, as it is stated: “And they could not observe the Pesaḥ on that day” (Numbers 9:6). The Gemara infers: On that day they could not observe it; on the next day they could observe it. Although they would be purified at nightfall and would then be eligible to partake of the Paschal lamb, at the time of the slaughter and the sprinkling of the blood they were not yet pure. They asked whether the Paschal lamb could be slaughtered on their behalf. Apparently, they were obligated to perform the mitzva of burial of the corpse although it prevented them from fulfilling the mitzva of sacrificing the Paschal lamb, which is a stringent mitzva. This is the source for the principle that one engaged in the performance of a mitzva is exempt from performing another mitzva.