I am curious. If there is a contradiction between statements of Chazal found in the Talmud, how are those reconciled in light of contradictory science, and if a Halacha is derived from a non-scientific viewpoint expressed by Chazal, do we still follow that Halacha or can modern Poskim and Rabbinic authorities contradict Chazal?

Sources for the answer to this question would be awesome.

  • 4
    I think you mean "science contradicting Chazal"
    – shmosel
    Commented Nov 10, 2023 at 22:31
  • leimanlibrary.com/texts_of_publications/…
    – Mike
    Commented Nov 10, 2023 at 22:37
  • 1
    Weird no-one has asked this before, given it's a well known contemporary controversy
    – AKA
    Commented Nov 13, 2023 at 3:00
  • I heard that we largely disregard rashis opinon on how and where the bnei yisroel enter and exit the yam suf due to a better knowledge of the geography of the yam suf. So maybe if we are 100% sure(as we are by the shape of the yam suf) we can disregard that shitah.
    – yogazefish
    Commented Nov 14, 2023 at 5:46
  • Are you aware of any practical examples? Commented Nov 14, 2023 at 8:22

2 Answers 2


This is a very broad and complex topic, with multiple opinions in every direction, and the practical halacha depends on the topic being studied. Entire books have been written on it, so it is not easy to summarize.

You could ask a separate question on each of the classical issues of disagreement (the mud-mouse, the louse's spontaneous generation, the path of the sun at night, the baby who cannot survive if born in the eighth month, etc.) and would find it being treated differently.

Depending on the specific issue, you will find various Rishonim and Acharonim disagreeing on the approach. Broadly they either

  • acknowledge the disagreement of science and gemara and resolve to keep halacha as is (most often)
  • acknowledge and suggest to change halacha to reflect the latest understanding (often for pikuach nefesh reasons)
  • reinterpret the gemara (sometimes in forced ways) to show it doesn't conflict with science or disagree with the science
  • claim science is wrong, or will be proven wrong

R Natan Slifkin wrote extensively on the topic (especially in his books Sacred monsters and Rationalism vs. Mysticism; Schisms in Traditional Judaism).

I think the best summary of his views is that one doesn't change the halacha even if discovering a scientific error for both practical and text-based reasons. He writes here

There aren’t so many cases of halacha being based on scientific error, and those that exist are fairly well known. Most famous is the Gemara’s permission to kill lice on Shabbat based on the mistaken belief that they spontaneously generate. In my book Sacred Monsters, I discussed various approaches that are taken. Some take the position that the halacha should be changed accordingly. But others (with whom I strongly agree) take the approach that the halacha remains in place.

The reason is that the halachic authority of the Gemara was canonized; we follow it even though in rare cases it may be based on mistaken beliefs. This is similar to the celebrated case of the oven of Achnai, where the objectively correct view - as attested by none other than God Himself - was overruled in favor of the majority. And the reason for this in turn is that stability is a crucial component of halachic authority. Such stability comes in some cases from following the majority, and it also comes from canonizing certain authorities and texts.

(The only exception is for cases where human life is at stake; thus, we do not follow the Gemara’s position that an eight-month fetus is less viable than a seven-month fetus, or that one may not clear away rubble on Shabbos to rescue someone who is not breathing.)

He brings other proofs here. See also here on MY.

  • 3
    I'd add an approach cited in the name of Rabbi EE Dessler that the halacha stands independently of the reason Chazal gave. For example, they had a tradition that lice may be killed on Shabbos, and were just giving a potential explanation they believed was true. In truth the reason may be a different one.
    – AKA
    Commented Nov 15, 2023 at 5:52
  • I agree, that was what I described (in different words) in my first bullet point. No different from when we say that when a reason is giving for a mitzva, even if the reason doesn't apply, we still apply the mitsva.
    – mbloch
    Commented Nov 15, 2023 at 14:39
  • Agree it falls under bullet point 1, but it's somewhat different to the reasoning you quote from Rabbi Slifkin below
    – AKA
    Commented Nov 15, 2023 at 22:44

Rabbi Yehudah HaNasi said that the Sages of Israel were mistaken in their view that the sun travels behind the sky at night. There is broad support from Rishonim and Acharonim on this. See the Gemara in Pesachim 94b.

  • 1
    But that is resolved within the works of Chazal (the Talmud) - no different to an argument between Beis Shammai and Bais Hillel. The OP's question stands for statements which are left standing by the Talmud but contradict later science.
    – AKA
    Commented Nov 12, 2023 at 14:48
  • 1
    @AKA You bring a good point.
    – Turk Hill
    Commented Nov 12, 2023 at 18:44
  • 1
    @TurkHill Yeah with all due respect I’m asking more for a source on when a statement in the Talmud is contradicted by modern science. Sources appreciated
    – Kirk
    Commented Nov 12, 2023 at 21:02

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