Shiur and sources available: here.
This week I discussed one of the thorniest issues in modern psak: What does Halacha do when the scientific assumptions it is based on turn out to be false?
In the background is a Gemara that tells of a dispute between the Jewish sages and the nations of the world about the correct path of the sun. The Jewish sages, after hearing the arguments of the non-Jewish sages, retract their position and admit the non-Jewish sages were correct. This Gemara implies that under some circumstances Chazal could be wrong about science. The question is whether this can be true in Halachic cases, and if so, what are the implications?
The locus classicus for this issue in the Gemara’s assertion, according to the opinion of the Rabbanan, that lice can be killed on Shabbos because they do not procreate. They are produced through spontaneous generation. R. Elazar agrees with the scientific assumption, but argues that anything that lives, whether it procreates or not, cannot be killed on Shabbos. In a second Gemara, Chazal assume that any premature baby born in the 8th month from conception is not viable, and Shabbos can therefore not be violated for that baby. In yet a third case, Chazal list different defects that deem animals unviable, tereifot. In all of these cases, the scientific assumptions of Chazal were incorrect.
The poskim have a range of approaches. On one extreme, you have those who deny the science. This position is taken by R. Yehuda Brill, the Rebbe of R. Yitzchak Lampronti in Pachad Yitzchak, as well as the late Lubavitcher Rebbe (who adds that not having seen spontaneous generation does not prove it does not exist). On the other extreme, R. Yitzchak Lampronti notes the Gemara about the path of the sun, and asserts that we have no ideological reason to believe Chazal were correct about science. Therefore, we can assume they erred and reverse the Halacha. R. Brill countered that everything Chazal said was divinely inspired, and thus they can never be wrong. He goes as far as to deny that Chazal were wrong about the sun’s path, claiming that later Chazal were proved right. [Amazingly, therefore, he claims Chazal were incorrect in their assumption that they could be incorrect.]
In the middle are many possible positions. For example, R. Aryeh Carmel cites R. Dessler as claiming that the Halacha remains the same because the Halacha is not actually based on the science. He claims that the Halacha predated the explanation, and Chazal were just trying to explain the Halacha they knew to be correct. This seems unlikely, as at least in the case of the lice, there is a dispute in the Gemara about the Halacha. R. Herzog, in a teshuvah about killing germs, suggests that Halacha didn’t mean that lice can be killed because they actually spontaneously generate, but rather because they seem like they do to the naked eye (in the case of bacteria this is true, but for lice this is highly questionable.) Another approach is to assume our science is correct, but Chazal were not wrong, because נשתנה הטבע – nature has changed. Thus, the assertion is made that 1) Chazal cannot be wrong but 2) we listen to our science. This is suggested by the Shevet HeKehati. Yet another approach is to assume that Chazal were wrong about science, but the Halacha remains the same due to formal reasons. The Dor Revii notes that many Rishonim explain that we listen to Chazal even when they are wrong about Halacha so the system can survive. The same should hold when Chazal were wrong about the science that which the Halacha is based on. [This comparison is far from obvious, as maintaining the legal principles but changing the facts is much more true to Chazal than abandoning their legal principles.] Yet another approach is to claim ignorance, being machmir unless there is a pressing need. This position is taken by R. Elyashiv, who allows killing lice if it really bothers a child. [R. Shlomo Zalman is cited by his students as saying the Halacha does not change, but R. Zalman Nechamia Goldberg and R. Nevenzhal provide different explanations of his position.]
However, while some people suggest taking these models for all cases – 1) Denial of Science 2) Acceptance of Science and Changing Halacha or 3) Reunderstanding the law, most poskim take a case by case basis. Thus, while many poskim will rule that you cannot kill lice on Shabbos, they will assume, as the Rambam did, that the list of treifot is based on Halacha LiMoshe MiSinai and cannot change. This was accepted by R. Moshe, Chazon Ish, and many others.
At the other extreme, few poskim will accept that a baby born in the eight month should not be saved. Either they accept this science was wrong, claim it is a safek and in cases of doubt we always save lives, or provide technical explanations, such we cannot know when the baby was conceived so we always save the baby misafek. They can also claim that the baby in not Halachically alive, but can be saved because by saving it, it will reach the status of alive (parallel to Ramban’s explanation of why Bahag thinks you can save a fetus – חלל עליו שבת אחת כדי שישמרו שבתות הרבה). In the Encyclopedia of Jewish Medical Ethics, Steinberg notes that this is the overwhelming consensus of poskim.
Thus, the case by case analysis seems to be the truest to the sources. For those who are interested, נריה גוטל provides seven rules for when we apply rules such as השתנות הטבע in his incredible work. The references for this book and several important resources are provided on the source sheets linked next to the shiur on YUTorah.