Banned V – Hazal and Science

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The issue of Hazal and science is more difficult than the others that we have tackled in this series simply because the literature is so vast. Where do I begin? As I stated originally, the main point of these series of posts is to make the argument that these are issues of contemporary debate, i.e. there are rabbis on both sides of this debate. However, because history is so important to us, I have quoted and will continue to cite ancient sources as well as contemporary.

Additionally, this current essay is omitting discussion of how we are to relate to this topic. If Hazal made scientific errors, how are we to relate to them? This is extremely important but space constraints force me to omit such a discussion. See the bibliography listed towards the end of this post for discussions of this important matter.

The issue for this post is the science of Hazal. Did they use their contemporary science, much of which has been disproven, or did they have Sinaitic traditions for all their scientific views?

On this matter, the medieval sources are quite prevalent. Let me make it clear that there was no single, unanimous opinion on this matter and that I am intentionally only citing sources on one side of this issue. There were certainly those on the other side as well. Be that as it may, the point still stands that there were rishonim whose views are important for this discussion.

Primary among the rishonim on this subject is the Rambam. In his Mishneh Torah (Hilkhos Kiddush Ha-Hodesh 17:24), the Rambam wrote:

The reasons behind all these calculations… and the methods by which they are derived belong to the science of astronomy on which the sages of Greece have compiled many books

The scientific and mathematical bases for the calculations of Kiddush ha-hodesh were, according to the Rambam, determined by the science prevalent during the times of the Sages. They were not, writes the Rambam explicitly (read further there), received traditions. Similarly, he writes in his Moreh Nevukhim (3:14):

You must, however, not expect that everything our Sages say respecting astronomical matters should agree with observation, for mathematics were not fully developed in those days: and their statements were not based on the authority of the Prophets, but on the knowledge which they either themselves possessed or derived from contemporary men of science. (Friedlander translation)

Before the Rambam, too, R. Sherira Gaon (or possibly his son, R. Hai Gaon) wrote similarly of the medical knowledge of Hazal (Teshuvos Ha-Geonim – Harkaby, no. 394):

Our sages were not doctors and said what they did based on experience with the diseases of their time. Therefore, there is no commandment to listen to the sages [regarding medical advice] because they only spoke from their opinion based on what they saw in their day.

R. Avraham ben Ha-Rambam wrote similarly in his Ma’amal Al Aggados Hazal (in Margoliyos edition of Milhamos Hashem, pp. 84-88), as did many other rishonim. For example, as one reader sent me, the Meiri to Horiyos 13b writes that Hazal‘s advice on how to avoid memory loss was based on what they found in “the books of the doctors.” I am sure that a thorough review of the Meiri’s writings will reveal many similar statements. Abarbanel, in his introduction to part 2 of Yeshu’os Meshiho (p. 17b), writes that Hazal‘s knowledge of science was based on their experience in their particular climate and time which might not apply to our climates and times (a forerunner of the “nishtaneh ha-teva” approach). That is in the rishonim, although there are certainly those who disagree (e.g. Rashba and Rivash).

The debate on this issue continued throughout the ages and remains to this day. For example, the Gemara (Pesahim 94b) records a debate between the sages of Israel and the sages of the nation over the sun’s movement. The sages of Israel took the pre-Ptolemaic view that the sun travels from east to west under the sky during the day and back again from west to east over the sky at night. The sages of the nations maintained the Ptolemaic view that the sun travels above the earth during the day and below (i.e. around) the earth at night. While the simple reading of the continuation of this passage is that the sages of Israel submitted to the proofs of the sages of the nations, Rabbeinu Tam was of the view that the sages of Israel never conceded their stance. Indeed, his entire approach to the onset of night is based on the pre-Ptolemaic understanding of the sun’s movement. While those who maintain that Hazal had received traditions for their scientific views will side with Rabbeinu Tam that the sages never conceded their position, the Maharam Al-Ashkar (no. 96) wrote explicitly that Rabbeinu Tam’s position is based on incorrect astronomy. Even the Minhas Cohen (4:10), a great advocate of Rabbeinu Tam’s position, admitted that its basis is on incorrect science but argued that this was irrelevant to the argument. (Note that Copernicus disproved both the pre-Ptolemaic and the Ptolemaic views; and see the citations from R. Yosef Qafah further.)

Moving to more current times, again skipping large sections of the literature due simply to its vastness, it is worth noting that R. Israel Meir Levinger, in his Sefer Ha-Ma’or Le-Masekhes Bekhoros (7b), corrects a Baraisa that states that bats lay eggs. In actuality, writes R. Levinger, the retired rabbi of Basel and a world-renowned expert on shehitah, bats do not lay eggs. I am sure that there are more examples but this one was pointed out by R. Shlomo Sternberg in his review of R. Levinger’s book in B.D.D. 4, Winter 1997 (p. 82).

R. Samson Raphael Hirsch wrote an essay titled “Trusting the Torah’s Sages” that was first published by Dr. Mordechai Breuer in 1976 in the journal Ha-Ma’ayan. It was translated into English by Yehoshua Leiman for Light magazine and then published in a booklet titled “Two Giants Speak” in 2002 by Neve Yerushalayim. R. Hirsch explained that Hazal relied on the science of their contemporaries and that many of their scientific statements have their origin in those sources. An example he brings is the statement in Bava Kama 16a that seven years after a person’s death, his spine turns into a snake unless he bowed at modim. This seems quite outlandish but is mirrored in the writings of Pliny. Clearly, R. Hirsch says, Hazal‘s source for this statement of fact was their contemporary science.

R. Eliyahu Dessler was of the following view, according to his close student and editor, R. Aryeh Carmell (B.D.D. 6, Winter 1998, p. 57):

Rabbi Dessler, with his commonsense approach, sought neither to deny scientific facts nor to assume drastic changes in nature since the time of Hazal. He accepted that Hazal did not get their ideas on nature from revelation, but from their cultural environment.

This approach is seen in Mikhtav Me-Eliyahu, vol. 4 p. 355 n. 4 in which R. Dessler is quoted as stating that, contrary to the claims of the Gemara, wildcats do not contain venom in their nails, streams are not warmer at night because the sun passes under the earth and lice are not generated spontaneously.

One of the most recent debates of this nature revolves around DNA testing. The Gemara (Nidah 31a) states that a person’s blood comes from his mother. Therefore, one could conclude that a DNA comparison of a child’s and his father’s blood cannot determine relation. And so do some posekim rule (e.g. Tzitz Eliezer 13:104). R. Yitzhak Herzog (in a letter published in Assia 5), however, argued forcefully against this position because science has clearly demonstrated that such matching works. Hazal, he states, had no received tradition on this matter and science has disproven the Gemara’s statement.

R. Yosef Qafah, in a number of places, also argues similarly. In his notes to Moreh Nevukhim 2:8, he comments that the above-mentioned debate between the sages of Israel and the sages of the nation is of interest mainly to “Archaeologists of Astronomy” because the science is outdated (see also his notes to 2:24). He writes similarly in his notes to Mishneh Torah (Hilkhos Yesodei Ha-Torah ch. 3 n. 1). In his notes to MT Hilkhos Shabbos (ch. 11 n. 4), R. Qafah states that the Gemara was incorrect about the spontaneous generation of lice.

R. Eddie Reichman, in the most recent issue of Jewish Action, wrote a glowing review of Mysterious Creatures (reprinted here with permission) and provided a good bibliography on this topic. I would only add to it R. Yehuda Levi’s Ha-Mada She-Ba-Torah, recently translated as The Science in Torah (Feldheim: 2004). Also, see my essay here.

To summarize this long but insufficient post, there is a clear history to claims that Hazal‘s scientific knowledge was based on the theories current in their time and can be disproven by later scientific discoveries. This debate continues to our time, with scholars on both sides of the disagreement.

About Gil Student

Rabbi Gil Student is the Editor of, a leading website on Orthodox Jewish scholarly subjects, and the Book Editor of the Orthodox Union’s Jewish Action magazine. He writes a popular column on issues of Jewish law and thought featured in newspapers and magazines, including The Jewish Link, The Jewish Echo and The Vues. In the past, he has served as the President of the small Jewish publisher Yashar Books and as the Managing Editor of OU Press. Rabbi Student currently is serving his third term on the Executive Committee of the Rabbinical Council of America and also serves as the Director of the Halacha Commission of the Rabbinical Alliance of America. He serves on the Editorial Board of Jewish Action magazineand the Board of OU Press. He has published four English books, the most recent titled Search Engine volume 2: Finding Meaning in Jewish Texts -- Jewish Leadership, and served as the American editor for Morasha Kehillat Yaakov: Essays in Honour of Chief Rabbi Lord Jonathan Sacks.

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