Five Ways of Reading a Gemara That No Longer Matches the World And How the Baalei HaTosafos Read Theirs

New York (VINNEWS/Rabbi Yair Hoffman)  Anyone who learns Shas runs sooner or later into a sugya in which Chazal describe the natural world in a way that seems to not quite match either what we see in front of us or what the secular world tells us. A classic flashpoint is killing lice on Shabbos. The Gemara in Shabbos 107b permits it on the assumption that lice do not reproduce through a form of pru urvu but emerge from sweat and dust. Modern biology disagrees. So what does a ben Torah do with that?

Over the centuries, Rabbonim have approached the question in roughly five different ways. Some communities lean heavily on one approach; others rotate between them depending on the sugya. Each has its defenders among gedolim, and each has its detractors. None of them is a casual position to hold.

The Five Approaches

1. The science is wrong. On this view, Chazal had access to truths—whether through tradition, ruach hakodesh, or simple observation in their own time—that modern science has not yet caught up to. If the Gemara says lice spontaneously generate, then they spontaneously generate, and the burden is on the scientist to prove otherwise. Voices in this camp range from those who treat every word of Chazal as scientifically infallible to those who simply refuse to be cowed by a scientific consensus that has, after all, reversed itself many times.
2. The Rabbis were wrong. This is the most controversial of the five. Its earliest and most famous proponent is Rabbeinu Avraham ben HaRambam in his Maʼamar al Derashos Chazal, where he writes that Chazal were not infallible in matters of medicine and natural science and were entitled to rely on the best science of their own day. [As a parenthetic note, one of our recognized Gedolei Torah was behind the printing of this Rav Avrohom ben haRambam, and he received a phone call from Rav Aharon Kotler zatzal—about to castigate him for doing so. The Gadol instructed his talmid to tell Rav Aharon that he wasnʼt home. The Talmid did so, but Rav Aharon responded, “Put him on the phone right now!”] Regardless, Pachad Yitzchak (Rav Lampronti) cites this approach in connection with killing lice. Many later authorities reject it sharply—some calling it close to kefirah—while others, including significant Sephardic poskim, treat it as a legitimate, if narrow, option. As will be seen below, this is the one category about which one might reasonably hesitate.
3. The halacha is right; the science attached to it is incidental. Here the move is to separate the ruling from the rationale. The din stands—lice may be killed on Shabbos—because it was given to us as a halacha lʼMoshe miSinai or fixed by the Sanhedrin, and the spontaneous-generation language was the way Chazal described what they saw in front of them. The science is, on this reading, a window-dressing on a halachic conclusion that does not depend on it. The author personally heard this approach from Rabbi Dr. Moshe Tendler z”l.
4. Nishtaneh haʼteva—the metzius has changed. This is the approach made famous by the baalei haTosafos. Chazal described their world accurately; what they described, however, is no longer the world we live in. Applying this approach, lice in their day really did emerge in a way that we, today, would not recognize. Cows in their day really did wait three years to give birth. Animal anatomy in their day really did look different. We are not contradicting the Gemara; we are reporting on a creation that has, since their time, changed.
5. A redefinition of what Chazal meant. On this newer approach, championed by such writers as Rabbi Moshe Meiselman and others, Chazalʼs categories—“louse,” “sheretz,” “reproduces,” “alive”—may not map cleanly onto the categories of modern biology. When Chazal said the louse eino poreh vʼrabeh, they may have been using a halachic-phenomenological category rather than a biological one. The microscope did not exist; the question of whether a louse “reproduces” in the modern molecular sense was not the question they were answering. What looks like a contradiction between Chazal and science may simply be a category mismatch.

Each of these approaches has been taught and held by serious Rabbonim. The author’s own view is that most of them deserve respect, and that a thoughtful Jew should feel free to choose the one he or she finds most compelling—with the possible exception of #2, which has historically generated more heat than light. The point is not to flatten the question but to recognize that Klal Yisrael has always had room for multiple legitimate ways of holding the tension between Torah and the world.

With that landscape laid out, it is worth taking a careful look at how the Baalei HaTosafos handled the question. Their answer is not the only one. But it is one of the earliest and most influential, and it shows clearly what option #4 looks like when it is taken seriously.

How the Baalei HaTosafos Approached the Problem

In three places in Shas and on three different topics, the Baalei haTosafos go on record that nishtaneh haʼteva—nature itself has changed since the time of Chazal. In each case there is a real-world observation driving the move; Tosafos are not invoking the principle in the abstract.

Fish freshly caught (Moed Katan 11a). The Gemara records, in the name of Ada the fisherman, that kavra samuch limsricha maʼali—the fish is best eaten close to the time of its salting, rather than the moment it is pulled from the water. Modern food science actually maps this onto a real phenomenon. A freshly killed fish enters rigor mortis within hours; during rigor, the muscle is taut and the texture is poor. As the fish ages and enzymes begin to break down the proteins, the flesh softens and the flavor develops. A short delay between catch and consumption—the kind that natural salting and transport once enforced—really does improve the fish. Eat it too early, and you are eating it in rigor; wait too long, and bacterial spoilage takes over. There is a window. Ada the fisherman, by all accounts, knew that window.

By the time of Tosafos, however, the empirical reality on the ground had reversed. People in their day reported that fresh-caught fish was the safe option and that fish that had sat for a while was the one that made you sick. Whether the relevant species had changed, whether the methods of preservation had changed, or whether human bodies had become more vulnerable to whatever toxins or parasites were involved, the practical equation no longer matched what the Gemara had described. Tosafos (s.v. kavra) write: “In this time people consider it dangerous to eat fish immediately upon being caught… perhaps the natures have changed, just as the medical remedies recorded in the Talmud are no longer effective in our day.” Either the species of fish itself shifted, or the human bodies eating it shifted; either way, the Gemaraʼs guidance no longer matched the world the baalei haTosafos lived in.

When a cow first gives birth (Bechoros 19; Avodah Zarah 24b). The Mishnah in Bechoros 3:1 rules that a cow purchased from a non-Jew, if it is under three years old, may be presumed to have never given birth—so any offspring it bears is a bechor. The premise is biological. A cow, on the Mishnahʼs picture, simply does not reach reproductive maturity early enough to calve before its third year; if it has a calf at age three, that calf is, by definition, the first. Modern animal science would treat this as a question of age at “first parturition”, which depends on breed, nutrition, climate, and management. Ancient Near Eastern cattle—smaller-framed and less intensively fed than what we would recognize today as dairy stock—may indeed have reached calving age later than the cattle that the baalei haTosafos saw in medieval Europe. Selective breeding alone, even before the modern era, can pull the average age of first calving down by a year or more.

Tosafos in Avodah Zarah point to what was, in their day, an everyday observation: cows were routinely seen giving birth at age two. Their answer: “It must be that the times have now changed from how they were in the early generations.” In earlier generations, Tosafos hold, cows really did wait three years; in their own day, the gestational and reproductive maturity of the animal had shifted. The biology had moved.

The lobes of the lung (Chullin 47a; Avodah Zarah). In the laws of treifos, the Gemara discusses the inunisa dʼvarda—the rose-shaped lobe of the lung—and treats it as an unusual feature, present only in some animals. The basic anatomical question is how many lobes a kosher animalʼs lung is supposed to have, and which deviations render the animal a treifah.

A modern veterinarian would describe a cow lung as having a fixed lobar pattern—cranial, middle, and caudal lobes on each side, plus an accessory lobe—with the small extra lobule, what shochtim call the varda, varying in size and prominence from animal to animal and even from breed to breed. When Tosafos opened the chest cavity of the cattle in front of them, however, they found that the rose-shaped lobe was not unusual at all—it was standard equipment in every animal they examined. Tosafos in Chullin note that no proof can be drawn from the animals available to them, since their vascular structure differs from that described in the Gemara, and Tosafos in Avodah Zarah state plainly that “nowadays this external lobe is found in all our animals.” Cow anatomy itself, they conclude, has shifted between the time of Chazal and their own time.

Three points are worth highlighting about how Tosafos handle these cases.

First, Tosafos do not merely use nishtaneh haʼteva as a way to defend the honor of the Gemara in the abstract. They actually allow the change in nature to reshape practical halacha. Once cows are observed to give birth before age three, the absolute presumption that any first birth at age three is a bechor no longer holds; the kohen would now have to bring positive proof of firstborn status, and one would also have to be concerned about the possibility of crossbreeding. Tosafos take the change seriously enough to let it move the line of practical psak.

Second, Tosafos could easily have avoided the whole nishtaneh haʼteva claim. The Mishnah in Bechoros can be read in another way: not that cows biologically cannot give birth before three, but that we are simply unwilling to assume two births within so short an interval. Had Tosafos adopted that mehalech—which Rashi in fact uses elsewhere—there would have been no need to invoke a change in nature at all. The fact that Tosafos rejected that reading as forced, and chose instead to suggest an actual shift in the natural world, tells us how seriously they took the empirical evidence in front of them.

Third, Tosafosʼ approach was not unanimous, even among the Rishonim. The Tosafos of Rabbeinu Elchanan (Avodah Zarah 62b) extends and embraces it: “It seems to me that nowadays one finds animals giving birth within two years of their own birth, and we find in the Talmud that the natures have changed from what they once were—just as in the case of the inunisa dʼvarda, which exists in all our animals, whereas in their animals it was found only in a baraisa.” But others hesitated, and the Yaavetz (Shʼeilas Yaavetz I:81) argued forcefully against the very move, holding that early births are simply rare (milsa dʼlo shechicha) and we follow the rov—the majority. Even within the camp that, broadly, accepts approach #4, there is a genuine spread of opinion about how aggressively to deploy it.

What This Tells Us

The Tosafos material is not a knockdown argument for nishtaneh haʼteva as the only legitimate response to the science-and-Chazal question. It is, however, a powerful illustration of what option #4 looks like in the hands of Rishonim who are committed both to the integrity of the Gemara and to honest engagement with the world they live in. They say neither that the science is wrong, nor do they say Chazal were wrong. They say, simply, that the world Chazal described was the world Chazal saw, and that the world has since shifted.

The author would also point out that Rav Dovid Feinstein zatzal’s approach in conversation, as well as Rav Yisroel Belsky zatzal’s approach, were not to make waves about the five different approaches but that those working in Kiruv should present the various approaches, explaining that each have supporters and detractors.

The author can be reached at [email protected]