Shinuy HaTeva

יב ואין להוסיף על טריפות אלו, כלל: שכל שיארע לבהמה או לחיה או לעוף חוץ מאלו שמנו חכמי הדורות הראשונים, והסכימו עליהן בתי דיני ישראל--אפשר שתחיה. ואפילו נודע לנו מדרך הרפואה, שאין סופה לחיות. [יג] וכן אלו שמנו ואמרו שהן טריפה--אף על פי שייראה בדרכי הרפואה שבידינו שמקצתן אינן ממיתין, ואפשר שתחיה מהן: אין לך אלא מה שמנו חכמים, שנאמר על פי התורה אשר יורוך

Here the Rambam is paskening that whatever treifos are listed in the Gemara we cannot add nor subtract from them. Even if we find something else that renders an animal physically a treifah we cannot add to the list. We should assume that the animal may be able to live for 12 months. Likewise, if we find that one of the treifos in the list in the gemara no longer renders an animal physically a treifah - it still has the halachic status of a treifah.

Why is this?

In an article in The Journal of Halacha and Contemporary Society (Volume 31 - Shinuy HaTevah) Rabbi Dovid Cohen brings a some opinions from the poskim on this issue. The Chazon Ish (Y.D. 5:3 and E.H. 27:3) says that it is basically like a gezairas hakasuv that only these treifos (that existed at the time of the gemara) should count as treifos l'doros. R' Moshe Feinstein (E.H. 2:3:2) says a similar idea but he adds to it. He asks why does the gemara need to say that the list of treifos are a halacha l'moshe misinai? He answers that it is specifically to tell you that this is the final list.

Based on his reasoning, R' Moshe had an interesting psak regarding the issur of petsua daka. He ruled that a certain procedure that was assured in the gemara (because it made one a petsua daka according to the gemara) is permitted nowadays because nowadays it doesn't render one a petsua daka. By petsua daka there is no halacha l'moshe misinai like there is by treifos that would say we follow the actual list in the gemara. Rather, by petsua daka we follow the actual metsius of what the procedure is determined to cause nowadays.

Interestingly, the Rashba (1:98) is cited by Rabbi Cohen as saying the following on the issue:


[The Rashba] quotes various instances where the Gemara recounts stories of people who claimed that they experienced circumstances which contradicted the rules of the Gemara. The Rabbis were persistent that the people were obviously lying about their stories and, in each ci:l.se, the people finally admitted that they had told the stories inaccurately. The Rashba concludes, therefore, that a statement made by the Gemara must always be considered true. Their teachings are all based on the teachings of Moshe Rabbenu; we are prepared to say even that one thousand people are lying rather than reject one word of the Gemara.

So according to the Rashba even if we think the metzius is different we should just assume we are wrong.

Rav Moshe however says that even the Rashba would have to admit in certain instances that the metzius is different if it is strikingly apparent. Rabbi Cohen cites some Gemaras that could fit into the "strikingly apparent" category:

1. Rambam Perush HaMishnayos Becohoros 8:2 - If a woman delivers a baby by C-section she won't be able to have another one.

2. Niddah 31a - Boys are born face down and girls face up.

*As mentioned, the material in this piece was taken from the much longer article in the Journal of Halacha. For practical halachic rulings consult a halachic expert.