Home    Introduction    Talmudic Issues    Essays  

 Pamphlets    Parsha    Original Essays    Contact Us 

Pamphlet 1 - Lice and Mice

Sages and Mistakes

The Gemara in Shabbat 107b rules that one is permitted to kill a louse on Shabbat because it does not sexually reproduce. This statement should be very troubling to the thinking student because, while the entire ancient world believed that animals could be created through spontaneous generation, we have known since the 17th century that this is incorrect. Animals that had been thought to be spontaneously generated were observed reproducing. With the aid of magnifying tools, lice eggs were found! How can an halacha be based on an observably incorrect fact? What does this do to our faith in the sages and the integrity of halacha?

These are all valid questions but we would be foolish to think that they had never been asked before. The great rabbis of the past few centuries were very much aware of this issue and we need to search for their treatments of this topic. For a longer analysis of this and similar issues, see our essay on Torah and Scientific Developments.

But first we must ask a basic question. Can the holy sages make mistakes? Granted, they poured all of their physical and mental energies into preserving and safeguarding the Jewish faith. But is it possible that with all this effort, with all the critical reviews and arguments by other rabbis anxious to remove all logical flaws, is it possible for an halachic ruling that was universally accepted to be mistaken?

The answer, the Torah (Leviticus 4:13-21) tells us, is yes. There is a special sacrifice to be brought when the elders rule mistakenly and cause others to sin. Indeed, the talmudic tractate Horayot is dedicated to the topic of mistaken halachic rulings. Only G-d is infallible. Mortals, however brilliant and dedicated they may be, are subject to error. Granted, the rigorous system of halachic review mitigates this possibility. But still, mistakes can occur. Despite this, the Torah (Exodus 23:2; Deuteronomy 17:10) commands us to follow the majority and fixes a severe penalty for a scholar who does not recognize his own limitations and refuses to follow the result of the halachic process (Deuteronomy 17:12). Even though the majority can err, we need to follow their decisions because without a process for determining halacha there would be no organized religion. Everyone, regardless of how qualified they are (or are not) to make an educated decision, would rule for themselves. There would be no unity and the Jewish community would collapse. As the Sefer HaChinuch (496) wrote, "It is better to withstand one mistake with everyone relying on one authority than to have each person follow his own halachic opinion because this would disrupt the religion, cause disunity of the people, and destroy the nation entirely."

Sages and Science

There is no question that the sages relied on scientists for knowledge of the world. This is not to say that everything the sages said was based on contemporary knowledge. However, as we show in our essay on Scientific Methodology in the Talmud, it is unquestionable that in at least some cases the sages utilized the science of their times. One obvious example is cited by the Chatam Sofer (Responsa, Even Haezer 1:30). The Gemara in Shabbat 85a quotes the ancient farmers who were descended from Seir HaChori as identifying how much soil a growth requires for sustenance. Evidently, the sages relied on testimony of ancient agronomists. About consultations with scientists, Rambam wrote in his Moreh Nevuchim (3:14):

Do not ask me to reconcile everything that they (the sages) stated about astronomy with the actual reality, for the science of those days was deficient and they did not speak out of traditions from the prophets regarding these matters.

The sages gathered some, but certainly not all, of their knowledge from the top scientists of their time. Sometimes, these scientists were wrong. This not a fault in the sages or these scientists. It is, however, an important piece of information we need to keep in mind.

Similarly in our case, there is no reason to believe that the sages were not influenced by the universal belief among scientists that lice were generated spontaneously. It was not foolishness or carelessness on the part of the sages. Quite the opposite. They would be subject to that accusation if they had ignored the opinions of all the scientists when their own observations confirmed the universally accepted theory.


It is clear that here we need to be willing to admit the fallibility of the sages. They were relying on the science of their times which has now been disproven. While a respect for halacha and the sages requires us to be very hesitant to change halacha, when the science has been undeniably refuted we must revise halacha. In our case, we are being stricter than the Talmud by prohibiting killing lice on Shabbat, an act the Talmud permitted. This also makes it easier to change halacha. Therefore, even after a healthily conservative approach, we must forbid killing lice on Shabbat. This was how R' Yitzchak Lampronti, the author of Pachad Yitzchak and the teacher of R' Moshe Chaim Luzzatto (Ramchal), and R' Yosef Kaffih approached this issue. See Pachad Yitzchak sv tzeida assura and Rav Kaffih's edition of Mishneh Torah, Hilchot Shabbat ch. 11 n. 4.

There is another approach, one that has been frequently misunderstood. There is a footnote to R' Eliyahu Dessler's Michtav MeEliyahu vol. 4 p. 355 that discusses this topic. The editor of this volume and author of the footnote, R' Aryeh Carmell, recently elaborated on his discussion with Rav Dessler that led to the footnote (Bar Ilan's BDD Journal 6, Winter 1998 p. ). According to Rav Dessler, the sages used science in two ways. One way was to understand the world based on which they made halachic rulings. For example, they observed which injuries were fatal to a man and therefore ruled that if a man had such an injury his wife could remarry within a year even if the husband's death could not be verified. In this case, where the science preceded the halacha, a change in science necessitates a change in halacha. If medicine now tells us that such an injury is not fatal then the wife cannot remarry until the husband's death is verified.

The second way in which science was used was to explain already existing halachot. In our case, this means that there was a tradition that one is permitted to kill lice on Shabbat. The sages, in their eminently logical ways, tried to explain this halacha based on the then-accepted scientific principle of spontaneous generation. Now that science has changed, the halacha does not need to change with it. The halacha preceded the science and the idea of spontaneous generationi was only an ex post facto explanation. If that explanation is incorrect then another, more suitable explanation should be sought.

Rav Dessler suggested that halacha is intended for people to follow and therefore only recognizes items that are visible to the naked eye. G-d does not expect us to rule halachically based on information we cannot naturally gather. Since lice eggs are too small to be seen unaided, lice look as if they grow from the item in which they appear and are given the same halachic status as their apparent origin. Since hair and fruit are not living animals that we are prohibited from killing on Shabbat (picking fruit off a tree is a separate prohibition), lice are given that same status.

613 Commandments

The Rambam in his Sefer Hamitzvot lists a number of prohibitions regarding eating forbidden foods. Among those, he listed a prohibition against eating insects that are generated from mold (prohibition 177). Since it is now clear that insects are not spontaneously generated from mold, this commandment is meaningless. Therefore, instead of there being 613 commandments there are 612. Has this scientific advance disproven the entire basis of the commandments?

The answer is no, for a number of reasons. First, it is impossible to prove that no insects are spontaneously generated. The most that can be proven is that all known insects do not spontaneously generate. Perhaps there are other insects that do not sexually reproduce and they are the subject of this prohibition.

According to Rav Dessler, this prohibition should remain also. The Torah commanded us not to eat insects that appear to us as if they grow out of mold.

However, this entire line of questioning is incorrect for a more fundamental reason. Even if we assume that no insects are spontaneously generated and that the prohibition is meaningless, this has minimal implications. To understand why, we need to know a little more about the "books of commandments" literature.

The source of this literature is the Gemara in Makot 23b:

R' Simlai exposited (darash): 613 commandments were given to Moshe at Sinai - 365 like the days of a solar year and 248 like the number of limbs in a person. Rav Hamnuna said: What is the textual source? "Moshe commanded us a Torah" (Deuteronomy 3:4). Torah is that in gematria (numerical equivalence). They asked: Torah is only 611 in gematria. [He answered:] That is because they heard "I am [the Lord your G-d]..." and "You shall not have unto you..." (Exodus 20:2) directly from G-d.

This, and a few other aggadic sources (e.g. Rashi on Genesis 32:5, Numbers 7:20, 15:39), is the basis of the count of 613 commandments (the number of knots in tzitzit is merely a custom; see Shulchan Aruch, Orach Chaim 11:14 and Bet Yosef there sv um"sh ela ikar). Ramban, in his comments to Rambam's Sefer Hamitzvot (p. 1) even suggests:

Perhaps this statement of R' Simlai is not unanimous but is disputed. R' Simlai counted the commandments as he saw fit and arrived at this number. He then established this textual exposition (drasha).

Everyone agrees that G-d commanded us many things. However, the classification of these commandments determines how many commandments there are. Should the daily morning and afternoon sacrifices be counted as one or two mitzvot? Are fearing and respecting one's parents counted as one or two? The classification and categorization of commandments is crucial for developing a count but it is also very subjective. It requires many uses of personal judgement that are, in the end, irrelevant.

The first endeavor to list all 613 mitzvot was the important geonic work Halachot Gedolot. R' Saadia Gaon also listed the commandments but his work was lost to us. R' Yerucham Fishel Perlow recently tried to recreate this list based on a piyut (liturgical poem) that R' Saadia Gaon wrote for Shavuot. The Rambam wrote his list in Sefer Hamitzvot and prefaced it with14 rules on classification. This Sefer Hamitzvot contains many disagreements with Halachot Gedolot. There are many places where Rambam removed a commandment that Halachot Gedolot listed and added a different commandment. Of course, they both agreed on what was commanded but they disagreed on how these commandments should be classified. Ramban rose to defend Halachot Gedolot and changed 34 commandments in Rambam's list. Other important lists of mitzvot were Sefer Mitzvot Gadol (Smag) by R' Moshe of Coucy and Zohar Harakia by R' Shimon ben Tzemach (Rashbatz) Duran.

With all this background, we can easily see that removing a commandment from Rambam's list is not a big deal. There are many other mitzvot that can take its place (according to Ramban, 34). Even if we did not replace it, it is possible that the entire concept of 613 commandments is a single aggadic opinion.


It has also been noted that the Gemara in Chullin 126b discusses a mouse that is half-flesh and half-earth. This topic is covered more fully in this essay. However, it should be noted that such a mouse was commonly reported in the ancient world. For example, Plinius wrote in his History of Nature, book 9 ch. 58:

For when as this river falls and returns again into its channel, a man may find in the mud young Mice half made, proceeding from the generative virtue of water and earth together: having one part of their body living already, but the rest as yet misshapen, and no better than the very earth.

Even in Rambam's time, there were reports of people seeing this mouse (see his commentary to Mishna Chullin 9:6 and R' Yosef Kaffih's analysis of those remarks in his edition of Mishneh Torah, Hilchot Shabbat ch. 11 n. 4). It is therefore unsurprising that the Talmud mentioned it. Should the sages have not believed the zoologists of their time? If they had not believed them, they would surely be criticized by many for rejecting science. They did believe them, and are criticized anyway. The sages listened to the scientists of their time, assumed the animal existed, and discussed its place in halacha. As discussed elsewhere, this theoretical discussion may have practical application with the advent of the ear-mouse.

After all these questions we are left with the simple truth that the sages could have made scientific errors based on the science of their time. But this does not mean that everything, or even many things, that they said are worthless. With a little analysis and a lot of humility, Torah and halacha continue. Whatever implications scientific advances may have, they do not merit rejecting the sages or their traditions.

Back Home

Contributor(s): Gil Student
Rabbi Nosson Slifkin www.zootorah.com
Last revised: 2/6/02
© Aishdas 2001-2002