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Va’eira: Changing Halachah in a Changing World?

This week’s parashah begins to chronicle the miracles in Egypt. Hashem overturned the entire order of nature when redeeming Israel from the Egyptian exile. Bnei Yissachar (Nissan no. 12) writes: “In the time of our redemption from Egypt, Hashem revealed to the world that He reversed the natural order of the heavens and constellations, when He bestowed His kindness upon His children.” In regular times, however, the order of nature, which Hashem directs yet does not interfere with, controls the world.

Many halachos in Chazal and the poskim are based on the nature of the world as perceived by our Sages. Many have questioned the status of such halachos, in light of our modern understanding of the world. How should we relate to the halachic rulings of Chazal and poskim that are at odds with modern scientific findings?


Laws of Eating Fish

The Gemara states that fish must not be baked together with meat in a single oven, because the one who consumes the fish may thereby contract leprosy. This principle is accepted by Shulchan Aruch (Yoreh De’ah 116:2). It is extended further (Shulchan Aruch, Orach Chaim 173:2) to the obligation to wash one’s hands between the consumption of meat and fish. “Danger,” concludes Shulchan Aruch, “is more severe than prohibition.”

Magen Avraham (173:1, cited by Mishnah Berurah 3), however, states that in our times, there is less danger in mixing the two, because nature has changed. For this reason, the halachah of washing one’s hands between fish and meat is not generally practiced. Shevus Yaakov (vol. 3, no. 70) writes that one should not rely on Magen Avraham, because one may not be lenient in matters of physical danger.[1]

Another instruction found in Chazal concerns the time one should eat fish. According to the Gemara, one should not consume fish close to the time it is caught, but rather eat the fish just before it goes bad. Tosafos state that in our times, the contrary is true: “In our days, it is considered a danger to eat fish right before it goes bad.” Tosafos explain this with a principle of dynamic nature: “It is possible that [nature] has changed, just as the medical treatments mentioned by Chazal cannot be applied today.”

A dynamic nature, at least regarding Talmudic advice of when to consume fish, obligates a dynamic halachah.


Other Changing Halachos

In fact, Rema himself (Even Ha’ezer 156:4) addresses the question of changing halachic rulings in the face of changes in the natural world.

Chazal established a physical fact that women do not give birth to healthy children in the beginning or middle of the ninth month of their pregnancy, but only at its end (although women can give birth to healthy children throughout the seventh month). Rema writes that the halachos derived from this principle no longer apply, because nature has changed.[2] The following are a number of additional examples where the principle of a changed nature is applied:

  1. The halachah states that a baby born in the eighth month of his mother’s pregnancy cannot survive. If born on Shabbos, it may not be treated or even moved (because he is considered muktzeh; see Shulchan Aruch, Orach Chaim 330:7). Today, poskim write that one must certainly do anything possible to aid a baby born during the eighth month of his mother’s pregnancy, for as Chazon Ish (155:4) notes, nature has changed, and the baby must be saved.[3]
  2. Shulchan Aruch (Orach Chaim 179:6) states that if one eats food without salt, or drinks a beverage without drinking water, he should expect bad breath during the day, and the danger of asphyxiation at night. In spite of this severe-sounding halachic ruling, Mishnah Berurah (18), quoting once again from Magen Avraham, writes that in our days we needn’t be cautious about this matter, because nature has changed.
  3. Chazal expound at length on the interpretations of dreams, offering a range of interpretations for numerous dreams. Shulchan Aruch Harav (288:7) writes that one cannot necessarily rely on these interpretations in cases that would lead to halachic ramifications. For instance, he writes that one should not fast on Shabbos based on a negative interpretation given by Chazal. The reason is that the meaning of dreams has changed, just as medical procedures have changed.


Inflexible Halachos: Tereifah and Kilayim

In contrast with the above there are some areas where we find halachic inflexibility. Thus, in spite of apparent changes in the natural reality, poskim write that the halachah does not change.

An example of this is the issue of treifos. Chazal delineate clear guidelines as to which injuries and physical defects render an animal a treifah, and which do not. The general principle is that a wound which can be healed does not render the animal a tereifah, and a wound that cannot be healed, causing the animal to die within the year, renders the animal a treifah.

Today, there are certain physical states that are curable, which according to Chazal render an animal treif. Does the fact that modern medicine is able to heal them, change the halachos of treifos?

Chazon Ish (Yoreh De’ah 5:3) answers this question negatively. The reason for the inflexibility of halachah, Chazon Ish explains, is that Chazal, in their Divine wisdom, established the laws of treifos based on the natural principles of their time—the last years of the ‘two thousand years of Torah.’[4] The laws of treifos are therefore immutable, and do not change as a result of advances in medicine.

Another example of the unyielding nature of some halachos is found in a responsum of Rav Kook (Mishpat Cohen, no. 14), concerning the laws of kilayim (mixing of the species). He rules that although the nature and classification of plants might change with the years, the principles governing which plants may be combined,  depend on the time of the Torah. Later changes in species do not affect these halachos.


The General Principle

Is there a general principle by which we can determine which halachos change with nature (such as the question of an infant born in the eighth month of pregnancy), and which do not change (such as tereifah and kilayim)?

Following a discussion of the laws of metzitzah (a mohel performing a circumcision sucks some of the blood from the bris into his mouth, and spits it out), Maharam Schick offers a principle.

The reason for the halachah of metzitzah, as given by Chazal and ruled by Rambam (Milah 2: 2), is in order to prevent potential harm to the baby. Today’s medical world, however, does not see the act of sucking blood from the bris as being healthy for the baby.

Nonetheless, Maharam Schick (Yoreh De’ah 244) rules that the mitzvah of metzitzah should be continued even today. He explains that the ruling of Chazal is based on tradition, and the doctors’ opinion is based on a ‘majority position’ i.e. whereas doctors are satisfied if in the vast majority of situations there is no danger, Chazal are not satisfied. Doctors’ guidelines, therefore, can not be followed in matters that can still be potentially dangerous to even a minute percent of the population.

Maharam Schick adds that even though we find that poskim do take into account the principle of a ‘changed nature,’ it does not affect matters that are halachah le-Moshe mi-Sinai—such as the laws of tereifos mentioned above. A similar concept is noted by Rav David Karliner (She’elas David, no. 1), who writes that changes in nature are not taken into account for halachos based on tradition, or halachos that are derived from Torah verses. Only those halachos that Chazal derived from the nature they knew, can be changed based on a changing nature.


Killing Lice on Shabbos

A discussion of halachah and nature cannot be complete without addressing the question of killing lice on Shabbos.

The matter of killing a louse on Shabbos is disputed among Tanaim. According to Rabbi Eliezer, “One who kills a louse on Shabbos is halachically equivalent to one who killed a camel!” There is no distinction, according to Rabbi Eliezer, between killing a louse, and killing a large animal; both involve an unequivocal desecration of Shabbos.

According to Chachamim, however, it is permitted to kill lice on Shabbos. The Gemara states that the reason is that lice multiply spontaneously from sweat or dust, and are not created by means of procreation (as other living things).

Tosfos describe the referred lice as the “black jumping louse,” and permits its killing on Shabbos. The “crawling louse,” however, is termed par’osh (flea), and it is forbidden to kill it, even according to Chachamim.

Shulchan Aruch (Orach Chaim 316:9) rules in accordance with the opinion of  Chachmim .Thus, killing lice on Shabbos is permitted, while killing fleas is forbidden. Mishnah Berurah (38) explains that the prohibition of killing living creatures on Shabbos is derived from the killing of ‘red rams,’ which were slaughtered in making the Tabernacle for the use of their skins. Since rams multiply by means of procreation, Chazal derive that it is only forbidden to kill creatures which multiply by means of procreation.


Do Today’s Lice Procreate?

Rav Yitzchak Lamfronti, was among the leading Talmudic scholars, of eighteenth century Italy. He questions (Pachad Yitzchak, Tzeidah, p. 21b) the modern application of this halachah. Science today believes unquestioningly that lice are born from eggs, just like other forms of life. Nature, as we know it, therefore appears to contradict the description of Chazal. The inevitable question thus arises; how can we rely on the halachic conclusion reached by Chazal, if the descriptive reality from which the conclusion was drawn, is apparently false?

Based on this postulation, Pachad Yitzchak rules that one may not kill lice on Shabbos, for it would violate a Torah prohibition.

Pachad Yitzchak expressed his wonder over the halachah of killing lice to his peer, Rav Yehudah Brill of Mantova. The latter responded, that in spite of the realistic question, “we may not change halachos that were established by ancient tradition, on account of discoveries made by the nations of the world. We may not stray from the halachic decisions of the Talmud, and even if the combined winds of human investigation blow against us, we will not yield, for the spirit of Hashem has spoken among us.”

Rav Lamfronti, however, expressed his disagreement with this position, stating that this statement runs contrary to basic logic. He continues that sometimes “the words of the Sages are based on human investigation, and not on tradition.” Therefore, in his opinion one must be stringent, and refrain from killing lice on Shabbos, because of the disparity between the assumptions made by Chazal concerning nature, and today’s knowledge of it.

In the twentieth century, the renowned mashgiach Rav Eliyahu Eliezer Dessler (Michtav Me-Eliyahu, vol. 4, p. 355, note 4) took issue with the reasoning given by Pachad Yitzchak. He explains that the reasoning mentioned by Chazal is not always the only reason for halachic principles, and we must follow their rulings even if we do not understand them. We are expected to search for alternative explanations for the rulings of Chazal. Even if we fail to find any, their dictates should be followed faithfully.


The Continued Discussion

The halachic discussion over killing lice on Shabbos has continued into the modern era, and present-day authorities remain in dispute over the halachic status of lice. However rather than focusing the debate on whether or not Chazal’s concept of spontaneous generation is plausible, the focus is rather on the question of changing nature: if our lice are unlike those of Chazal, there is perhaps room for stringency.

Rav Nissim Karelitz (Chut Shani, Shabbos, chap. 15) has ruled that one should adopt the stringency of Pachad Yitzchak, and refrain from killing lice on Shabbos—a position also stated in Shevet Hakehasi (vol. 3, no. 126) in the name of Rav Elyashiv.

Birur Halachah (part 4) discusses this issue at length, noting that Az Nidberu of Rav Binyomin Silber, and many others, rule that one may follow the Shulchan Aruch and kill the lice found in hair even today. He adds that this is the halachic position of Rav Chaim Kanievsky.

Vayizra Yizchak points out that even plants ‘procreate,’ and that Chazal were well aware of this phenomenon. What, in view of this, is the meaning behind the Talmudic statement whereby lice do not procreate? He responds that Chazal meant to state that it is only forbidden to kill those creatures that are able to procreate of their own accord, independent of external aids. Lice, however, are only able to multiply by means of an enzyme released by the human scalp (the enzyme is released by children to a greater degree than adults, explaining the greater frequency on lice among children), which is the reason why there is no prohibition of killing them on Shabbos.



  • There are certain halachos that depend on physical nature. Where nature has changed these halachos have changed. Examples are caring for a child born in the eighth month of pregnancy, eating fish that are almost spoiled and eating salt after partaking of food items.
  • Many authorities maintain that rulings of Chazal that result from perceived dangers cannot be readily changed. This is the view of the Shevus Yacov concerning eating fish and meat and the Maharam Shick concerning metzitzah.
  • Certain halachos are immutable. Examples discussed are the Chazon Ish‘s ruling concerning treifos and Rav Kook’s ruling about kilayim.
  • A further debate involves matters in which current scientific opinion contradicts the physical reality as described by Chazal. The example discussed is killing lice on Shabbos.

[1] It is interesting that Rema, and poskim that follow in his wake, refer to a question of physical danger, whereas the Gemara only mentions a concern for leprosy.

[2] Note that this approach gives rise to a leniency concerning laws of yibum.

[3] It is interesting that Mishnah Berurah does not make any note of the possible change in nature, and accepts the ruling of Shulchan Aruch at face value. The change in nature was perhaps less evident in the time and place of Mishnah Berurah, leading him to accept the fact of 100% infant mortality among babies born in the eighth month.

[4] The Gemara teaches that the total number of years of this world is six thousand, which divides into three sets of two thousand years. The first two thousand years are years of chaos; the second are years of Torah; the third are years of Mashiach (meaning years in which the Mashiach can come). The early era of Tanaim coincides approximately with the ending of the two thousand years of Torah.

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