Chullin, Chapter 2, Mishnah 6. Throughout this chapter we have been working through what makes a shechita valid, and here the trouble arrives from a different direction: not the knife or the cut, but the condition of the beast. An animal can be so feeble that we honestly cannot tell whether life had already left it before the blade finished its work. Should it have expired even an instant earlier, the shechita accomplished nothing, the carcass is a neveilah, and it is forbidden as food. What we require, then, is proof of two things: that life was still present when the throat was severed, and that the severing is what brought that life to an end. The mishnah supplies the signs by which such proof is furnished.
The case: hashochet et hamesukenet
"Hashochet et hamesukenet," one who performs shechita on an animal that finds itself in sakanah, in danger: it is so far gone that its legs will not hold it up, and if someone sets it upright it sinks down again. Be precise about what is not under discussion. We are not speaking of a treifah, an animal doomed by an injury or a defect in one of the organs on which life depends; a treifah stays forbidden no matter how flawlessly the shechita is carried out. This animal, by contrast, is structurally intact. The one worry is that its soul may have slipped away a heartbeat before the knife arrived, leaving us with the meat of a creature that had already died.
Rabban Shimon ben Gamliel: movement of a limb
Rabban Shimon ben Gamliel sets the bar: "ad shetefarkes bayad uvaregel." Our default assumption is that death came first, and that assumption is overturned only if the animal jerks a leg, foreleg or hind leg, while the cut is being made. Life expresses itself in reaction. A twitch during the shechita tells us the creature had not yet expired, so the knife is what took its life and the meat may be eaten. A vigorous animal, after all, kicks and struggles because it feels the pain; here too, whatever life remains will announce itself through motion.
Rabbi Eliezer: the spurting of blood
"Rabbi Eliezer omer, dayo im zinkah." For Rabbi Eliezer it is enough that the blood shoots out as the throat is being cut. Sever the neck of a living creature and the blood leaves under pressure, in a jet. That jet, he holds, testifies to life all by itself, and no visible twitching of the limbs need be observed.
Rabbi Shimon: a practical application
"Amar Rabbi Shimon." Rabbi Shimon now extracts a practical ruling from Rabbi Eliezer's standard. "Af hashochet balailah, ulemachar hishkim umatza kotlim melei'im dam, kesheirah, shezinkah, uchemidat Rabbi Eliezer." Someone slaughtered after dark, when there was no light by which to watch for a twitch, and at daybreak he came and found the walls beside the animal's neck splashed with blood. The animal is kosher. Blood in that quantity and in that place could only have shot out as the cut was finished, and such spurting testifies that life had not yet departed, precisely by the yardstick Rabbi Eliezer gave us.
The Chachamim: a middle position
"Vachachamim omrim, ad shetefarkes o bayad o baregel, o ad shetechashkesh bizenavah." The Sages offer a third view. The animal demonstrates that it was alive through one of three movements: a convulsion of the foreleg, a convulsion of the hind leg, or a wagging of the tail. They are therefore more lenient than Rabban Shimon ben Gamliel, who insisted specifically on a leg and would not accept a wag of the tail as proof, and more stringent than Rabbi Eliezer, who was satisfied with spurting blood alone, which the Sages do not accept. Their opinion stands between the two.
Small animals and large animals
"Echad beheimah dakah ve'echad beheimah gasah," the law reads the same for a small animal, a sheep or a goat, and for a large one such as a cow. Whichever it is, a beast that had been failing is forbidden to us unless its living state at the hour of the slaughter has been established beyond doubt. Still, the two categories are not identical in one detail.
"Beheimah dakah shepashtah yadah velo hechezirah, pesulah, she'einah ela hotza'at nefesh bilvad." Should a sheep or a goat push out a foreleg and leave it extended without drawing it back, the shechita does not count, since a small animal makes exactly that motion by itself while the soul is leaving it. That is the exiting of life, not a signal of life. An ox behaves otherwise: it does not thrust out a limb in that fashion as it dies, so in a large animal the very same gesture does serve as testimony that the creature had not yet expired.
Only for an animal presumed to be dying
"Bameh devarim amurim, shehaytah bechezkat mesukenet." All of the above, the demand that a slaughtered animal display some sign of life before it may be eaten, is confined to a beast that already bore the status of one whose life was ebbing away when it came to the shechita. "Aval im haytah bechezkat beri'ah," if instead the animal was reckoned a healthy one, then "afilu ein bah echad" (even lacking a single one) "mikol hasimanim halalu, kesheirah." Though it never so much as stirred while the cut was completed, there is nothing here to trouble us. Its life is not in question, and the animal is kosher.