Can new discoveries in science and advances in technology bring about changes in halakha? The question is not whether halakha can address phenomena that did not exist in the time of the Talmud, such as electricity, surrogate motherhood, and organ transplants; that is the regular work of halakha. The question, rather, is whether a halakha formulated on certain scientific or technological assumptions can change once those assumptions are proven incorrect. The Gemara, for example, states that a baby born in the eighth month of pregnancy is not viable and that her mother cannot even nurse her on Shabbat because the baby is “like a stone” and will definitely die. This is in contrast to a baby born during the seventh month, which the Gemara considers to be viable. In another case, the Gemara makes it forbidden to eat meat and fish cooked in the same oven because it poses a health hazard. Since we now know it to be otherwise, should the halakha change to reflect our current knowledge?
A number of people would object to the notion that the Rabbis of the Talmud could make errors in science. Rambam was certainly not bothered by this; he wrote that the Rabbis’ possessed scientific knowledge no more advanced than the scientists of the time (Guide to the Perplexed, II:8 and III:14). Others not prepared to concede this point but unable to deny that their direct experience of the world ran contrary to statements in the Talmud argued that nature had changed since the time of the Talmud: nishtaneh ha’tevah (see, for example, Tosfot Avoda Zara 24b, s.v. parah). Either way, once it was accepted that reality was not as the Talmud described, the question arose: Will halakha change as a result? The answer has implications for many halakhot and mitzvot, two of which appear side by side in our parasha. In the first, the Torah admonishes us regarding a number of people who cannot enter into “the congregation of the Lord,” that is, who cannot marry another Jew. One of these is the petzuah dakah, the man with crushed testicles; another is the mamzer, the person born from an illicit union (23:1–2).
In February 1963, Rav Moshe Feinstein was asked about the case of a man who had a testicular biopsy so that the doctors might determine why he had been unable to have children (Iggrot Moshe Even Ha’Ezer 2:3). If any part of the testicle was removed, the man would be considered a petzuah dakah according to the Gemara, and he would be forbidden to continue living with his wife. Rav Moshe noted that the procedure in question could quite likely help—and it certainly would not hurt—the man’s fertility. Thus, he concluded, if it could be established, first, that the Gemara’s determination was not based on the physical condition of the organ alone but on the assumption that such a condition made the man sterile and, second, that the Gemara’s ruling could be reassessed based on current scientific knowledge, we could then conclude that the man would not be a petzuah dakah.
This led Rav Moshe to analyze at length the question of whether halakha can change with new scientific knowledge. This had actually been discussed extensively through the centuries with the issue of treifot, animals with injuries considered to be fatal. Rambam ruled that the list of injuries cannot be updated based on new medical knowledge, even to be more strict (Laws of Shechita 10:12–13). This point was passionately reiterated by Rashba in a responsum (1:98), as it has been by many poskim since. But Rav Moshe argued that the case of treifot was an exception to the rule, being that the treifot were ultimately known and concretized through tradition and not science. For other halakhot, the matter was different:
[F]or we find in many other cases that the Torah relied on the Rabbis’ assessment of reality, regarding absorption and transfer of taste [of foods in vessels], and when a planting takes root, and similar issues….[And when it comes to matters other than treifah,] the determination is based on the assessment of the doctors of any given time….We thus see that unless we are compelled otherwise, we should assume that matters that are dependent on nature should be based on the assessment of the rabbis of every given time.
For Rav Moshe, any halakha based on an assumption relating to science or the natural world can be reassessed as our knowledge changes.
Does this mean every halakha should be reassessed on this basis? The answer is no. The process of changing halakha based on science can be threatening and disruptive; acknowledging error can serve to undermine faith in the authority of the Rabbis or the divinely-binding nature of the system. Allowing science to dictate halakhic change also locates ultimate authority outside of the system, with science and scientists and not with the Rabbis; this is why Rav Moshe spoke about the determination of the Rabbis and not scientists. Beyond all of this, change is disruptive. Any legal system must be fundamentally conservative: the law must be stable so that it can support, guide, and direct behavior. No posek worth his salt is interested in doing a wholesale audit of halakha to determine which halakhot are out of sync with science to then change them accordingly.
The opposite—that no halakha should ever be revisited—is equally not true. A good posek knows that sometimes the law must be flexible; it must be able to respond to the human condition. The ability to reassess a halakha based on science can be an effective tool in finding halakhic solutions to challenging cases. Thus, Rav Moshe used his principle to rule that the man is not a petzuah dakah, but he did not use it to reassess the laws of kashrut, which he could have easily done, and with good reason. Today’s pots are made from stainless steel, and they don’t absorb the taste of non-kosher food. If we were to reassess the laws of absorption of taste, we would wind up jettisoning half the laws of kashrut. Rav Moshe wisely lets that possibility lie dormant. (Interestingly, just this week Rav Eliezer Melamed of Yeshivat Har Bracha reawakened that possibility, arguing that after the fact, food cooked in a clean stainless steel pot is always kosher, regardless of what it was used for in the past!)
The balance between stability and responsiveness can also be seen in the cases mentioned at the outset. In the case of a baby born in the eighth month, with a human life at stake, almost all poskim state that the ruling of the Gemara is no longer operative; the baby is considered viable and Shabbat must be broken to protect his life. However, there is no major need to reassess the prohibition of cooking meat and fish in the same oven, so that halakha remains.
This brings us to the second mitzvah, the prohibition against a mamzer marrying anyone who is not a mamzer. In July 1977, Rav Waldenberg dealt with the question of child support: In a case where the paternity of the child was in doubt, could a blood test be used to demonstrate that a particular person could not be the father? Rav Waldenberg argued that halakha could not recognize the results of such a test, inasmuch as the Talmud states that the red matter in a person, including the blood, comes from the mother and not the father. To argue this way seems nonsensical: the Talmud passage in question isn’t a halakhic ruling, and there is no question about the science behind a blood test. But Rav Waldenberg knew what he was doing. To have allowed a blood test to be used in halakha would mean that we could determine that someone’s father was not the man married to his mother, in other words, that the person was a mamzer. This would be highly disruptive to the system, which goes to great lengths to minimize cases of mamzeirut, and disastrous in terms of the human cost.
The introduction of change into the system brings about consequences, both seen and unforeseen, and it is just as likely that it will make things worse rather than better. The ability to reassess halakha based on science is a powerful tool in the hand of a posek, and it must be wielded responsibly. A good posek is one who knows that halakha must be as responsive as possible to human needs and that it must remain stable, consistent, and true to our mesorah. While different poskim will strike this balance differently, it is on every posek to ensure that our Torah remains both a Torat emet and a Torat chayim.