The baraita (Hullin 33a) teaches: “One who wishes to eat the flesh of an animal before it expires, may cut a piece from the exposed throat of the animal, and after the animal expires he may eat that piece of flesh.” This baraita teaches us that not only does shehitah prevent an animal from becoming a neveilah, it removes the prohibition of eiver min ha-hai, immediately rendering the animal’s flesh fit for consumption, even while the animal, convulsing after shehitah, is considered to be alive. However, the Gemara (Id.) states that for Gentiles (who are also forbidden to consume eiver min ha-hai) nehirah does not render an animal fit for consumption until after death.

That nehirah is not effective until the animal dies is implied by the verse “ושפך את דמו” (from which R. Akiva infers that, in the desert, nehirah was a valid method of slaughter) and by the prohibition against eating flesh with its soul (“אך הבשר בנפשו לא תאכלו”) and (“לא תאכל הנפש עם הבשר”), implying that, once the lifeblood has been drained from an animal, eating from the animal no longer violates the prohibition of eiver min ha-hai. Because the Scripture identifies blood with the life force of the animal (“כי הדם הוא הנפש”) extracting the lifeblood is required for the animal’s halakhic status of being alive to change, involuntary convulsions not being considered a sign of life after extraction of the animal’s lifeblood. The Rambam codifies what constitutes the lifeblood of an animal in Hilkhot Tumat Ohkhalin (10:3):

The blood that is counted as a liquid is the blood that flows from a kosher domesticated or undomesticated animal or fowl at the end of the process of slaughter. Blood that flows at the beginning of the slaughter, by contrast, does not make food susceptible to ritual impurity, because the animal is still alive.

In other words, after the blood starts to flow from the initial wound, the continuing flow of blood causes the loss of the animal’s life force, so that only the blood flow associated with death is that blood which renders an object susceptible to contracting ritual impurity, and that blood flow may occur even if the animal is still convulsing involuntarily. Nehirah therefore became effective only at the moment of death, not with the initial loss of blood, which is why the verse “ושפך את דמו” was understood to refer to a blood flow from a place from which most of the animal’s blood would be shed – i.e., the simanim, but not the heart — and causing blood to flow from that place is what is called nehirah. Thus nehirah, of which there is no explicit mention in the Torah, could not be effective unless – and until – it caused the lifeblood to be drained.

Moreover, the animal must die within 24 hours (מעת לעת) of the nehirah, a requirement inferred from Exodus 21:20-21 about a Canaanite slave (“אם מת תחת ידו ואם יום או יומיים יעמוד לא יוקם”) which the Sages understood to mean 24 hours. Accordingly, if death must be caused by nehirah, then a time elapse greater than 24 hours between the act of nehirah and death means that the death cannot be attributed to the nehirah, so that the corpse would be a neveilah. However, shehitah is effective immediately, even before death. Thus, shehitah involves a significant leniency, but the source from which this leniency is inferred is not obvious.

The difference between nehirah and shehitah in respect of when they become halakhically effective is also disputed by R. Yishmael and R. Akiva, because the verse “בכל אות נפשך תאכל בשר,” from which R. Yishmael infers that, before entry into the Land, b’sar ta’avah was forbidden, seems redundant for R. Akiva. But it cannot be that R. Akiva would derive no halakhic inference from a textual redundancy. Thus, R. Akiva maintained that, when nehirah was a valid method of slaughter, anyone desiring to eat the flesh of an unconsecrated animal did not have to wait for the animal’s death before eating its flesh, because killing an animal immediately by cutting off its head satisfied the requirement of nehirah. Qodshim, however, may not be eaten without a valid shehitah, and cutting off the animal’s head would invalidate the shehitah, the distinction between shehitah and nehirah being that shehitah requires the simanim to be sliced without applying downward pressure while the application of downward pressure to sever the simanim is allowed when performing nehirah.

So, in the desert, someone with no pressing desire to eat the flesh of an animal immediately would perform shehitah and bring the animal as a peace-offering; shehitah and the other sacrificial rites having been performed, the sacrificial meat would be eaten from the Heavenly table. However, anyone with an immediate, pressing desire to eat meat could perform nehirah by cutting off the head of the animal, instantly causing its death, so that the flesh of the animal could immediately be eaten. But when the Torah, upon entry into the Land, prohibited nehirah as a method of slaughter, thereby sanctifying us with the obligation to perform shehitah on hullin, even those who were near the Tabernacle, but desirous of eating meat immediately, and unwilling to wait for the sacrificial rites to be performed, were permitted by the Torah, as a concession to their desire, to take the animal’s flesh immediately by first performing shehitah and then cutting off its head.

Thus, in prescribing the commandment to perform shehitah on hullin, the Torah addressed two distinct groups of people. The passage “כי ירחק ממך המקום . . . ואמרת אוכלה בשר” was directed to the group that, but for the distance from the Tabernacle, would have brought the animal as a sacrifice. But there was a second group consisting of those, unwilling to wait for the sacrificial rites to be performed, wanting only to satisfy an immediate desire to eat meat — those who would say “I want to eat meat” (אוכלה בשר) because of an urgent desire to eat meat (“כי תאוה לאכול בשר”). Those in the second group are also told by the Torah to perform shehitah as they were commanded (“וזבחת כאשר צויתיך”) rather than perform nehirah. However, to accommodate their desire to eat meat, the Torah allowed them to eat the meat of the animal immediately after shehitah is performed, by saying “בכל אות נפשך תאכל בשר,” implying that meat may be eaten immediately after performing shehitah, because, after shehitah is performed, the animal’s head may be cut off, killing it instantly.

But permission to eat the flesh of an animal immediately after shehitah was granted only as a concession to the evil inclination, just as the permissibility of a female captive in war was a concession to the evil inclination. The Scripture therefore concludes “ואכלת בשעריך בכל אות נפשך,” implying that there were two permissions granted: first, to eat hullin on which shehitah was performed within your gates (i.e., wherever you live, even far away from the Tabernacle); and second, that one whose desire to eat meat is so powerful that he can’t wait for the animal to die may eat the animal’s flesh immediately (“בכל אות נפשך”) without waiting till shehitah causes death.

The foregoing discussion accords with the opinion of R. Akiva that a positive commandment to perform shehitah, accompanied by the leniency that shehitah is immediately effective, was instituted upon entry into the Land. However, according to the opinion of R. Yishmael that nehirah was never valid, so that shehitah was always the only valid method of slaughter whereby the status of neveilah could be prevented from devolving upon an animal at death. In the desert, therefore, b’sar ta’avah was prohibited, becoming permissible only upon entry into the Land through the verse “בכל אות נפשך תאכל בשר”. Thus, according to R. Yishmael, there is no textual source from which to infer the effectiveness of shehitah even before an animal’s death. For R. Yishmael, therefore, the effectiveness of shehitah is exactly analogous to the effectiveness of nehirah in the desert for R. Akiva; shehitah prevents the status of neveilah from devolving upon an animal at death, but does not nullify the prohibition of ever min ha-hai until death.

According to R. Akiva, the minimum requirements for a valid shehitah (cutting both simanim — trachea and esophagus – of a four-legged animal, and one siman — trachea or esophagus – of a fowl) were not introduced until entry into the Land, as Rebi inferred from the verse (Deuteronomy 12:21) “כאשר צויתיך” that Moshe received as an oral commandment. The alternative derivations of these requirements that are provided at the beginning of chapter “Ha-Shohet,” having been brought by tannaim who followed the opinion of R. Yishmael and interpreted the verse “כאשר צויתיך” as a reference to the prior obligation to perform shehitah on qodshim, were rejected by Rebi. Instead, following the opinion of R. Akiva, Rebi understood “כאשר צויתיך” to refer to a new halakhah le-moshe mi-sinai specifying minimum requirements for a valid shehitah of four-legged animals and of fowl, and he must have also held the opinion that, in the desert, there had been no minimum requirements for a valid shehitah. Because shehitah in the desert did not become effective immediately, but only upon death of the animal (within 24 hours) after the shehitah, a minimum requirement for a shehith to be effective had been unnecessary, because as long as the animal died from the loss of lifeblood after the shehitah it did not matter how much of an animal’s simanim had been cut. It was only, upon entry into the Land, after the halakhah had been changed to make shehitah effective immediately, that prescribing the minimum amount of the simanim to be cut became necessary to create a halakhic presumption that the animal would die within a short time after shehitah was performed.

Only with this introduction can we understand R. Akiva’s opinion in the Mishnah (Hullin 32a) that a defective shehitah, in which one siman was cut properly and the other punctured, renders the animal a tereiphah (whose flesh is prohibited) but not a (ritually unclean) neveilah. R. Akiva’s reasoning before he retracted his opinion in favor of R. Yesheivav’s opinion that any defective shehitah renders an animal a neveilah is difficult to fathom. One might conjecture that R. Akiva, who believed that, inasmuch as nehirah was a valid method of slaughter in the desert, despite being invalidated as a method of slaughter upon entry into the Land, it still retained a certain residual validity to prevent the status of neveilah and its associated ritual uncleanliness from devolving upon animals slaughtered thereafter by that method.

However, this supposition is refuted from the text of the Mishnah which says “או שחט אחד מהן,” so R. Akiva must have believed that at least one siman had to be properly cut for an animal not to be rendered a neveilah. Thus, even before retracting his original opinion, R. Akiva could not have believed that nehirah, which required neither siman to be cut, retained any validity, because even if “כאשר צויתיך” meant only that a positive commandment to perform shehitah on hullin had been introduced, the negative commandment of “לא תאכלו כל נבלה,” which was also transmitted in the Mishneh Torah (Deuteronomy), implied that the status of neveilah would devolve on animals slaughtered by the formerly valid method of nehirah. Because this new prohibition of neveilah took effect after entry into the Land, only tereiphah, which also subsumes neveilah, was mentioned in parshat Misphatim. That the prohibitions of tereiphah and neveilah are identical is codified by the Rambam (Hilkhot Ma’akhalot Assurot (4:17)) who explains that, unlike other prohibitions derived from different verses, separate quantities of neveilah and tereiphah are combined to constitute the minimum quantity required for lashes to be administered. Thus, the term ”tereiphah” must include not only a live, but fatally injured, animal, but also a corpse (neveilah).

The question then arises: why, in parshat Re’eh, when the Torah explicitly mentions the prohibition of neveilah, but meant only to repeat the prohibition of tereiphah in parshat Mishpatim, was “neveilah” mentioned instead of “tereiphah,” inasmuch as “tereiphah“ includes both tereiphah and neveilah? Apparently troubled by this question, the Sifre posits that the inclusive term “כל” in the verse “לא תאכלו כל נבלה” teaches us that tereiphah, not just neveilah, is included in the prohibition, which we would not have known had the text stated “לא תאכלו נבלה”. The Sifre is actually consistent with its opinion elsewhere, because, as shown in Iqar 1, the Sifre, explaining the verse “וזבחת כאשר צויתיך,” understands “צויתיך” to refer to requirement to perform shehitah on qodshim, following the opinion of R. Yishmael that the requirement to perform shehitah became effective in the desert, so that, after the Torah was given, an animal slaughtered by the method of nehirah had the status of neveilah. That is why the Sifre could interpret of the verse “לא תאכלו כל נבלה” in Deuteronomy as a reiteration of the earlier prohibition of tereiphah, which included both neveilah and tereiphah. The prohibition was repeated to teach that, upon entry into the Land, the preferred method of disposing a neveilah resulting from a defective shehitah (e.g., nehirah) was to give it either to an observant Gentile resident of the Land or to sell it to a foreigner. Only in the desert did the commandment “לכלב תשליכון אותו” (to throw the neveilah to the dogs) apply, inasmuch as there were no non-Israelites in the desert, to whom a neveilah could be given or sold.

But according to R. Akiva, who held the opinion that nehirah was effective in the desert, a new and distinct commandment concerning neveilah had to be given before entry into the Land to teach that an animal slaughtered by the method of nehirah, whose flesh would, in the desert, have been permissible, would, upon entry into the Land, be considered neveilah;  without a new prohibition, the flesh of an animal slaughtered by nehirah would not have been neveilah, so that the flesh of such an animal would have been prohibited only as an issur asseih (not punishable by lashes) inferred from transgressing the positive commandment (“וזבחת“) to perform shehitah on hullin. A tereiphah upon which shehitah is performed before its imminent death from its injuries, or a neveilah that died without being slaughtered even by the method of nehirah must be thrown to the dogs, a tereiphah and such a neveilah being fit only for dogs, not human beings created in the image of God. But the new category of a neveilah resulting from a nehirah or an invalid shehitah is fit for human consumption, and may be given to the Gentile residents of the Land or sold to foreigners. Thus, according to R. Akiva the explicit prohibition of neveilah in Deuteronomy must have applied to nehirah or to an invalid shehitah.

It appears therefore that R. Akiva’s understanding that the verse “וזבחת . . . כאשר צויתיך” introduced a new commandment to perform shehitah on hullin was derived from the difficulty in interpreting the verse “לא תאכלו כל נבלה.” Why, in parshat Mishpatim, did the Torah express the prohibition in terms of “tereiphah,” but in parshat Re’eh the prohibition was expressed in terms of “neveilah”? At the beginning of Deuteronomy, the Ramban writes that the additional commandments mentioned in Deuteronomy, but not mentioned previously, had either not been observed at all, or only sporadically, in the desert. But why was the prohibition of neveilah not mentioned until the book of Deuteronomy? Moreover, how could the Torah have commanded that neveilah be given to an observant non-Jewish resident of the Land or sold to a foreigner, if the flesh of an animal that died of natural causes is unfit for human consumption, being universally prohibited by the laws and customs of civilized people, and the prohibition of neveilah was implicit in the prohibition of tereiphah in parshat Mishpatim? That difficulty led R. Akiva to explain that the prohibition in parshat Re’eh was introduced because of the new commandment to perform shehitah, thereby categorizing an animal slaughtered by the method of nehirah as neveilah.

The explanation for R. Akiva’s original opinion that cutting one siman of an animal properly suffices to prevent the animal from being rendered a neveilah must be that, according to R. Akiva, there had been no obligation, in the desert, to cut both simanim when performing shehitah on qodshim. Indeed, there was no reason to specify a minimum cut for the shehitah of qodshim to be valid, inasmuch as the basic requirement of a valid shehitah or nehirah is for the slaughtered animal to die within 24 hours after at least one siman has been cut or severed. Indeed, Rebi says explicitly that the requirement to cut one siman of a fowl and two simanim of a four-legged animal was not introduced until the commandment to perform shehitah on hullin was given. Before retracting his original opinion, R. Akiva must therefore have believed that, although after we were commanded to perform shehitah on hullin, slaughtering by the method of nehirah rendered an animal a neveilah, as implied by the verse “לא תאכלו כל נבלה,” the minimum requirement that two simanim of a four-legged animal be cut, which, had not existed even for qodshim in the desert, was meant only to prohibit the flesh of the animal to be consumed, not to render the slaughtered animal a neveilah and, thus, ritually impure. At first, R. Akiva held that the verse “וזבחת . . . ואכלת“ was meant only to prohibit the flesh of an animal unless slaughtered by a valid shehitah, but any animal upon which one siman was properly cut and then died within 24 hours as a result of that cut – a shehitah that had been valid in the desert — would not be rendered a neveilah, because the verse “לא תאכלו כל נבלה” was presumed to apply only if both simanim were not cut, but punctured. This was R. Akiva’s opinion until R. Yesheivav persuaded him to retract it by citing the tradition of R. Yehoshua that the halakhah that a shehitah is valid only if both simanim are properly cut applies to the prohibition of neveilah and not just to the positive commandment to perform shehitah.

This interpretation is entailed by the text of the Mishnah (Id.): “If he cut the trachea but punctured the esophagus, or even if he cut only one (either trachea or esophagus) and waited until the animal died.” The words “and waited until the animal died” (המתין לה עד שמתה) seem unnecessary, inasmuch as an animal contracts, and conveys, the ritual impurity of neveilah only after dying, because there is a Mishnah at the beginning of the chapter “Ha-Ohr Ve-Ha-Roteiv” (Hullin 117b) teaching that, even if one cut both simanim of an unkosher animal, the ritual impurity of neveilah does not devolve upon the animal until death, because the Scripture explicitly says “כי ימות מן הבהמה” (see Hullin 37a). The language of the Mishnah was therefore precise in specifying that only if the animal died from the cutting of just one siman does R. Akiva argue against the opinion that the animal contracts the ritual impurity of neveilah. But if the animal was slaughtered subsequently by an improper cut of the second siman, R. Akiva would agree that the animal is thereby rendered a neveilah and transmits ritual impurity.