Gemara

(Tana d'Vei Eliyahu): Six thousand years were decreed upon the world: 2000 years of emptiness (without Torah), 2000 years of Torah, and 2000 years for (the days of) Mashi'ach.

(Rav Papa): If a Chacham (knows the centuries, but) does not know the excess (how many years of Mashi'ach (i.e. after the year 4000) have passed), he should add 48 to the number of years written in Gitin.

Chulin 43a (Ula): Eight kinds of Tereifos were taught to Moshe on Sinai - punctures, cuts, missing organs, deficient organs, tears, Drisah (being clawed by a venomous animal), animals that fell, and broken organs.

Ula excludes afflicted organs, i.e. Rachish bar Papa's teaching about an afflicted kidney.

54a: Certain hunters would kill animals by hitting them on the Gid ha'Nasheh, or on the kidney.

(R. Yehudah ben Beseira, and R. Aba): This does not make an animal Tereifah. We cannot add to the Tereifos that Chachamim listed.

Even though we see that the animals die from this, we have a tradition that a salve could enable them to live.

Rishonim

Rambam (Hilchos Shechitah 10:12): One may not add to the Tereifos. Anything that happened to an animal or bird other than what the early Chachamim listed and Batei Din of Yisrael agreed upon, it is possible that it will live, even if we know medically that it will not live.

Kesef Mishneh: The Rambam learns from Chulin 54a. He explains that a tradition from Sinai teaches that a salve could enable them to live, even though our medical books say that there is no cure.

Darchei Teshuvah (29:3 DH uv'Inyan): Tevu'os Shor 33:1 says that if an animal cannot eat, obviously it cannot live! Chachamim did not need to list it among the Tereifos, for they already taught that whatever cannot live is Tereifah. Kanfei Yonah rejected this. Will we say that an animal is Tereifah if it ate poision, or if it has a muzzle that cannot be removed?

Igros Moshe (CM 2:73:4): Natiure has changed in many ways, like Tosfos (Mo'ed Katan 11a DH Kavra) says. Also sicknesses and cures can change. Tereifos were fixed permanently at the time of the Mishnah, and perhaps also in the time of the Amora'im. Tereifah was whatever could not live. All the details were given to Moshe on Sinai. Hash-m did not rely on Da'as Chachamim, like He did for many other things, for then Tereifah would be only what cannot live at that period of time. The law would change when nature changed! The rule that what cannot live is Tereifah was true at the time of Matan Torah, and also at the time of the Mishnah and Gemara. The Rambam connotes that an injury is not Tereifah because Chachamim knew that it can live, even if now we do not know it. I explained that even if nature changed and now there is no cure, it is Kosher! This is like he wrote in Halachah 13, that what Chachamim declared Tereifah is Tereifah even if nowadays it can live. It seems that the text is it was able to live. Nowadays everyone knows that many Tereifos live long lives.

Rambam (13): Everything that Chachamim listed is Tereifah, even if it seems from our medical knowledge that some are not mortal injuries and it can live. We follow only what Chachamim said - "Al Pi ha'Torah Asher Yorucha."

Rambam (Hilchos Ma'achalos Asuros 4:9): If an animal is about to die due its wounds and it cannot live, it is forbidden. From here, Chachamim said that whenever an animal wounded like this cannot live, it is Tereifah.

Ha'Emek She'elah (125 p.33): Why didn't the Rambam bring Reish Lakish's verse "Zos ha'Chayah"? The Rambam teaches that there are Tereifos that are known only because they do not live. The Mishnah's rule 'whatever cannot live...' does not teach about a Safek, like Reish Lakish said. Rather, it teaches about Tereifos that were not listed, e.g. the upper jaw was removed. We do not add a bruised kidney to the Tereifos, for it can be cured (Chulin 54a). If not, we would add it! However, it is not common and it is hard to say surely (that it cannot live). Even if we know medically that it cannot live, perhaps there is a cure of which we are unaware. We have a tradition about the 70 Tereifos that surely they cannot live. Even if doctors can cure it, it is Tereifah. The Shach (57:48) says that if it lives more than 12 months, this is a miracle. The Maharshal says that a tiny minority can live more than 12 months. Both of these are difficult. Rather, a Tereifah cannot live, i.e. by itself. It can live through slaves and cures. If a Safek Tereifah lives 12 months, we assume that it was not Tereifah, for it is very rare that it would chance upon something that heals it. If a Vadai Tereifah lived 12 months, we must say that this happened. This applies to Tereifos of tradition. If we have no tradition for it, it is Tereifah only if there is no cure and it is destined for dogs. We learn from "la'Kelev Tashlichun Oso." This is like the Rambam. The Ramban and Ran hold that wharever can heal was never Tereifah. The Rambam rules like Rachish bar Papa, that an afflicted kidney is Tereifah, and also like Ula, even though the Gemara said that he excludes Rachish's case! The Sugya is like Reish Lakish, who says that 'whatever does not live' explains (Safek) Tereifos of our Mishnah. The Gemara asked about the additions that the Amora'im added. They saw that they do not live, even with salves, so this clause includes them. We answered that Reish Lakish agrees that the clause includes them. Even so, Ula excludes Rachish's case (for there was no tradition for it). However, we hold like Rava, that the general rule of the Mishnah includes these other Tereifos, and they are like the 18 Tereifos of the Mishnah. No Heter for Tereifos can come through man, but b'Yedei Shamayim there is, according to Rashi and the Rosh, e.g. a puncture that became tangled with flesh, or a punctured kidney that was removed b'Yedei Shamayim. It is Tereifah as long as the problem is there.

Ra'avad (on Toras Kohanim Shmini, Sof Perek 3): The Toras Kohanim gives many Drashos to exclude Tereifos. These are Asmachtos, for the 18 Tereifos are a tradition from Sinai.

Ha'Emek Davar (Shmos 22:30, footnote 1 DH v'Hinei): What was the Hava Amina to expound to add to a tradition from Sinai? Rather, the Toras Kohanim first forbids a punctured Veshet or an uprooted Kaneh, for one cannot slaughter them. They are not Tereifos. Then it teaches that a cure does not permit a Tereifah.

Rivash (447): We do not judge Torah laws according to scientists and doctors. If we would, Torah would not be from Shamayim, Chalilah! They would change Tereifah animals to Kosher, and vice-versa. We say that if a k'Zayis remains of the liver, it is Kosher, and they say that it dies immediately. We forbid all lesions, and appearances that disqualify the lungs, and laws of Drisah. They mock us (for they say that it lives). May molten gold be poured in their mouths! The Rambam said that the juncture of sinews is in the lower bone in birds. Chachamim did not accept this, for the Gemara connotes that it is in the middle bone, and there is no distinction between birds and animals. Even though the Rambam was a Chacham in medicine and dissection, we do not rely on this. We rely on our Chachamim, even if they say that what is (apparently) right is really left, or vice-versa. They received the truth and passed it from generation to generation, from Moshe Rabbeinu. We do not rely on reasoning or experiments of Greeks or Yishmaelim, who were not mindful of Sefekos that can invalidate the experiment. 'I bring a proof from Torah, and you bring from fools!?' (Nidah 30b) They argue with Divrei Chazal in many ways. E.g. we say that a fetus is formed on day 40, and before this it is mere water. We say that the father supplies white semen, which forms the bones... and they say that everything is from the mother.

Me'iri (Chulin 54a DH Kol): They thought to add to the list of Tereifos, until they heard that one may not add. Similarly, if something is called a Tereifah, and we see that such an animal lives, even if we see this constantly, we are not lenient. Even if doctors testify about it, what Chachamim listed are the Tereifos; we are not lenient or stringent unlike their words.

Tosfos (Chulin 47a DH Kol): We cannot bring a proof from our animals. Perhaps they are different from those in the days of the Gemara. Bechoros 19b says that a calf born to a three year old cow is a Bechor (for cows do not give birth before this), and our cows give birth in the second year! To say that cows did not change, we would need to say that if it gives birth in the second year, it is weak and does not give birth again until the fourth year.

Chazon Ish (YD Tereifos 5:3 DH v'Af): Hash-m created cures even for Tereifos. Not all cures were known in all generations. Some were revealed and forgotten. Hash-m entrusted Chachamim to fix the Tereifos according to the Ru'ach ha'Kodesh that He put on them. It had to be fixed in the 2000 years of Torah, like it says in Avodah Zarah. Ravina and Rav Ashi are Sof Hora'ah (Bava Metzia 86a). We cannot be Mechadesh Torah after them. Tereifos were determimed according to Hash-m's providence at that time. The illnesses in the arsenal of the angel of death at that time, for which no cure was known, those are the Tereifos that the Torah forbade then and for all future generations. Also, nature has changed. E.g. bloodletting used to be medically beneficial. Nowadays, it is harmful. This is like the Bach and Taz on (EH 17:31), and unlike the Chelkas Mechokek and Beis Shmuel. The Ramban, Rashba, Ram, Ritva and Nimukei Yosef all say like I said; we do not find anyone who disagrees.