The Mishnah at the beginning of “Ha-Shohet” (Hullin 27a) teaches that cutting one siman is sufficient for a shehitah performed on a fowl to be valid while cutting both simanim is required for a shehitah performed on a four-legged animal to be valid, and cutting most of a siman is sufficient.
הַשּׁוֹחֵט אֶחָד בְּעוֹף וּשְׁנַיִם בִּבְהֵמָה – שְׁחִיטָתוֹ כְּשֵׁרָה, וְרוּבּוֹ שֶׁל אֶחָד כָּמוֹהוּ
The Gemara explains that the language of the Mishnah implies that the ideal (לכתחלה) requirement is to cut the entire siman, and that cutting a majority of a siman is valid only after the fact (בדיעבד). The בדיעבד terminology indicates either 1) that לכתחלהboth simanim should be cut when performing shehitah on fowl, or 2) that לכתחלה the entire siman (either of a fowl or a four-legged animal) be cut when performing shehitah. The apparent disagreement between the two opinions about the distinction between the לכתחלה requirement and the בדיעבד requirement for a valid shehitah is explained in three ways.
1 Rashi’s explanation: According to the first opinion that לכתחלה both simanim should be cut when performing shehitah on fowl, the requirement to cut both simanim is just a rabbinic stringency, lest not even a majority of one siman be cut. However, according to the second opinion that an entire siman must be cut לכתחלה though, בדיעבד, cutting a majority of a siman is sufficient the distinction between the לכתחלה and the בדיעבד requirements is itself Biblical. Consequently, according to the first opinion that the distinction between לכתחלה and בדיעבד refers to cutting one siman of a fowl the distinction is based on a Rabbinic stringency for which there is a rationale (to ensure that the majority of at least one siman is cut), while the second opinion provides no rationale for the distinction between לכתחלה and בדיעבד because the distinction is Biblical.
This explanation is incomprehensible. First, it is implausible that, because a majority of one siman might not be cut, the rabbis would have required, merely as a stringency, cutting both simanim לכתחלה. Second, according to Rashi’s explanation, the first opinion must have agreed with the second opinion that the לכתחלה there is a Biblical requirement cut the entire siman — the Mishnah stating explicitly one siman of a fowl and both simanim of a four-legged animal must be cut — so that a rabbinic stringency had already been imposed to at least cut a majority of the siman. If so, why would the Rabbis have instituted an additional stringency that לכתחלה both simanim of a fowl must be cut? But the first opinion is also problematic inasmuch as it concedes that cutting most of a siman is, only בדיעבד, like cutting the entire siman, so that the בדיעבד language of the Mishnah could simply have referred to the sufficiency of cutting a majority of the siman. Why then did the first opinion choose to posit unnecessarily that the Mishnah used בדיעבד terminology to teach that there is a לכתחלה requirement to cut both simanim of a fowl, a requirement not mentioned in any Mishnah or baraita?
2 The explanation of Rabbeinu Hoshaya in the Tosaphot (Hullin 30b ד”ה החליד): A majority of a siman is the לכתחלה requirement both for fowl and for four-legged animals, there being no difference between the לכתחלה and the בדיעבד Biblical obligations. This explanation can be inferred from his explanation that the question posed in the Gemara about pausing during shehitah while cutting the simanim (במיעוט סימנים) is referring to someone performing shehitah with a dull knife who pauses after having cut a majority of the first siman of a four-legged animal. In this case, the question is whether cutting a majority of the siman renders the siman as if it were completely cut, so that continuing to cut the as yet uncut portion of the siman is like cutting a limb of the animal, which would be considered an impermissible pause between cutting the first and second simanim. From that explanation it is evident that, according to Rabbeinu Hoshaya, the Biblical obligation is to cut only a majority of a siman, and cutting the entire siman is only a Rabbinic stringency. But if so, continuing to cut the remainder of the siman is like cutting a limb, because if the לכתחלה requirement to cut the entire siman were Biblical, it would be inconceivable that fulfilling the לכתחלה Biblical requirement to cut the entire siman could be considered a pause in the shehitah similar to cutting a limb of the animal?
A further difficulty, aside from those discussed above, with this explanation is that if cutting the simanim completely is just a Rabbinic stringency, why, according to the first opinion, was it necessary to adopt a further stringency that לכתחלה both simanim of a fowl must be cut? One might say that the first opinion believes that the requirement to cut the simanim completely is Biblical, in which case the dispute between the two opinions is whether the requirement to cut simanim completely is Biblical or rabbinic. If so, according to Rabbeinu Hoshaya, the question whether cutting the final small portion of the first siman constitutes a pause between cutting the first and second simanim would corresponds to the question raised in the Gemara whether a pause while cutting a siman invalidates the shehitah to be cut completely is Biblical or rabbinic.
3 The explanation of the Rambam: The requirement to cut simanim completely is Biblical, and even for fowl the Biblical obligation לכתחלה is to cut both simanim, as the Rambam writes explicitly in Hilkhot Shehitah (1:9): “What is the requirement for a shehitah? Two simanim, the esophagus and the trachea, should be cut whether upon a fowl or a four-legged animal, and one who slaughters should intend to cut both.” The two opinions cited in the Gemara do not disagree at all, but the Mishnah teaches that, בדיעבד, cutting just one siman of a fowl is sufficient and for either fowl or four-legged animals, cutting a majority of either siman is sufficient.
These three explanations differ in their understanding of Rebi’s interpretation in the baraita of ”כאשר צויתיך.” The Rambam and Rashi understand Rebi to mean that the commandment לכתחלה requires the trachea and the esophagus to be completely cut while the requirement to cut a majority of one in a fowl and a majority of both in a four-legged animal is a בדיעבד obligation. But Rashi understands that לכתחלה the requirement for a four-legged animal is to cut both simanim completely, but for a fowl the לכתחלה obligation is only to cut one siman completely. Rashi’s distinction between the לכתחלה obligation to cut two simanim in four-legged animals and only one siman in fowl is problematic, because the Mishnah should have explicitly mentioned that distinction. The Rambam’s understanding, however, is straightforward, the לכתחלה requirement being the same for four-legged animals and for fowl.
However, according to Rabbeinu Hoshaya that the לכתחלה Biblical obligation was only to cut a majority of one siman of a fowl and a majority of both simanim of a four-legged animal, the interpretation of halakhah taught by Rebi must be that Moshe was taught the location of the shehitah, meaning that shehitah must cut the trachea and the esophagus, with the requirement that more than half of the simanim be cut, to the exclusion of stabbing the heart or any other place from which blood could be drawn.
Whether a majority of a siman or the entire siman must be cut לכתחלה depends on the dispute between R. Yishmael and R. Akiva. While there is no difference between the לכתחלה and the בדיעבד requirements not to violate a Biblical prohibition, there are many examples of לכתחלה requirements for fulfilling a positive Biblical commandment that do not preclude fulfilling the commandment even if the לכתחלה requirement is not satisfied. For example, a childless widow is required to recite a statement if her brother-in-law opts not to perform a levirate marriage, halitzah being effective בדיעבד even if the statement is not recited. Similarly, the ideal requirement that the four species be taken as a bundle on Sukkot does not prevent one from fulfilling the commandment if the species are not bundled together.
According to R. Akiva, the verse “כאשר צויתיך“ shows that there is a positive commandment to perform shehitah before eating the flesh of an animal, a commandment that is supplementary to the need to eliminate the preexisting prohibition against eating the flesh of a live animal or of a dead animal whose death did not result from a valid shehitah. Consequently, there can be a difference between the לכתחלה obligation to cut both simanim completely and the בדיעבד obligation simply to cut a majority of both simanim of a four-legged animal and of one siman of a fowl. But according to R. Yishmael, shehitah is necessary only to eliminate the prohibition against eating animal flesh that is otherwise fit for human consumption, but fulfills no positive commandment to perform shehitah before eating animal flesh. So, according to R. Yishmael, there would be no reason to distinguish between a לכתחלה obligation and a בדיעבד obligation. And in accord with the opinion of R. Akiva and Rebi, the Rambam explicitly codifies a positive commandment to perform shehitah if one wishes to eat animal flesh.
However, the Ohr Same’ah on the Rambam writes that the distinction between a לכתחלה and a בדיעבד requirement does not apply to any commandment that serves only as a preparation for another commandment inasmuch as the only function of such a commandment is to enable the fulfillment of another commandment, there being no reason to add an additional requirement to whatever is sufficient to enable fulfillment of the commandment in question. But the comment of the Ohr Same’ah shows that, according to R. Akiva, the verse “וזבחת מבקרך ומצאנך” was intended to create a new obligation to perform shehitah before animal flesh may be consumed, and was not just a preparation to eliminate the prohibition against the consumption of animal flesh. On the contrary, the commandment to perform shehitah invalidated nehirah, rendering the flesh of any animal slaughtered by the method of nehirah neveilah. But precisely because nehirah had once been a valid method of rendering animal flesh halakhically fit for consumption, the commandment to perform shehitah may be performed both in the ideal (לכתחלה) way in which both simanim are completely cut and in a less-than-ideal (בדיעבד) — but still halakhically valid — way, by cutting a majority of one of the two simanim of a fowl and a majority of both of the simanim of a four-legged animal.
But this reasoning is relevant only to R. Akiva, who believes that nehirah was once a valid method of slaughter. According to R. Yishmael, who believes that shehitah was always the only halakhically valid method of slaughter, there could be no positive commandment to perform shehitah and therefore no distinction between a לכתחלה and a בדיעבד requirement, so that there is no halakhic preference for cutting both simanim entirely when performing shehitah.