Halakhah su Esodo 13:2

Dayan Grossnass cites a passage from the Gemara (Temura 25a) that seems to clearly disprove Rav Moshe’s contention. The Gemara teaches that when there is a conflict between an individual’s directive and Hashem’s directive, the latter prevails. For example, if one declares that when a firstborn animal will be born it will be a korban olah (burnt offering), Hashem’s designation of the animal from birth as a bechor (a firstborn, which must be brought as a sacrifice; Shemot 13:2) prevails over the individual’s desire. Similarly, Dayan Grossnass argues, if one stipulates that at death his assets should belong to a halachically unrecognized heir, Hashem’s directive that the assets belong to the halachic heir prevails. Rav Hershel Schachter remarked that he finds this proof particularly convincing.

The commandment of sanctifying the first-born in the Land of Israel: To sanctify the first-born; meaning to say that all those that are born first, which is to say [that] come out first from the womb of the female - whether with people or with beasts - the males are holy to God; as it is stated (Exodus 13:2), "Sanctify for Me every first-born; the first issue of every womb among the Israelites - man and beast - is Mine." And specifically the beast (behamah) - which is an ox and and a sheep and a goat - but not a wild animal (chayah). And from the impure beasts, only the donkey is [included] in this commandment (Bekhorot 10a). And the content of the commandment with a pure beast is that it is a commandment upon the owners to sanctify it and say, "Behold this is holy." And they are obligated to give the first-born to the priests (Kohanim); and [the latter] offer its fat and its blood on the altar and eat the meat in Jerusalem. And he does not give it immediately when it is born, but rather takes care of it - with [sheep and goats] for thirty days; and with [cattle] for fifty days (Bekhorot 10a). And outside the Land, where we do not have a Temple - according to some commentators (Mordechai in the name of Rabbi Eliezer MiMetz on Avodah Zarah, Chapter 1) - he locks the door in front of it and it dies on its own. But there are [others] of them (Mishneh Torah, Laws of Firstlings 1:10) that said we should always kill it. And if a blemish developed upon it, he can feed it to any man and in any place that the priest wants to give it to him. And it is considered like unsanctified meat (as it is like unsanctified meat) - as it is stated (Deuteronomy 15:22), "the impure and the pure shall eat it, like the gazelle and the deer." And so wrote Ramban, may his memory be blessed, in his Laws of Firstlings (at the end of Chapter 5). And we shall explain the topics of the first-born of a man and the [first-born] of a donkey with the commandment of the redemption of each of them, with the help of God. And they are in this Order and in the Order of Vayikach Korach.

The commandment of redeeming the first-born of a man: To redeem the first-born of man; as it is a commandment upon every man in Israel to redeem, from the priest, his son who is the firstborn to his Israelite mother - as it is stated (Numbers 18:15), "but you shall redeem the first-born of man." Elsewhere we find that the Torah makes being firstborn dependent on exiting the womb, as it it is stated in the Order of Bo el Pharaoh (Exodus 13:2), "all the first-born, all that exit the womb of the Children of Israel - whether man or beast - is Mine." And the meaning of exit the womb is opening the womb, meaning who was the first to open the womb of the mother. And because of this, they, may their memory be blessed, said (Bekhorot 46a) that one who is born after a stillborn - any stillborn for which the mother would be impure [as a result of it] - the one that comes after it is not a first-born to [be redeemed from] the priest, as the stillborn preceded him; but if the fetus [is not developed enough that] the mother [contracts] the impurity of birthing, the one that comes after it is a first-born. In Tractate Niddah (Niddah 21a) this distinction is elucidated, and likewise in Tractate Bekhorot (Bekhorot 46a) is elucidated which first-born children are considered first for the priest and not for inheritance, and which are first for inheritance but not for the priest. It also said there that there is a firstborn for everything and one that is not a firstborn for [even] one of them.