Shabbat 39a says: "With regard to heating food in the sun itself, everyone agreed that one is permitted to place food in the sun to heat it, as it is certainly neither fire nor a typical form of cooking".

However, today we know that sun and fire, from physics point of view, are actually quite similar (they consist of matter in form of plasma). So the statement "it is certainly not fire" can be questioned. That brings me to the following question:

Can halacha be changed/adjusted, if a scientific research shows that some of the argumentation behind it was actually not valid? Has it happened, does anyone know some examples?

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    I think that your final question is an excellent one, which is the subject of much debate by recent and contemporary authorities. However, the particular example you use to get to it is not a great one, since you're assuming that the word "fire," used in Talmudic discussion of Jewish law, means the same thing as the word "fire" used to describe a particular thermochemical reaction in contemporary scientific literature.
    – Isaac Moses
    Feb 3, 2014 at 19:57
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    Actually, cooking through the sun and fire are different. The sun heats the food through thermal radiation, while fire usually heats the food through conduction or convection. See enwp.org/heat_transfer.
    – Ypnypn
    Feb 3, 2014 at 20:04
  • Thanks for the comments, yes, I'm aware of this difference between sun and fire. However, the similarity between them which was probably not considered before, brought me to the final question I posted. As Isaac says, maybe it's not the greatest example; I'd be happy to know about some other (better) ones.
    – Mariana R.
    Feb 3, 2014 at 20:26
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    The most oft-discussed example of this issue is probably that of the permission to kill lice on Shabbat based on the understanding that they are spontaneously-generated. Here's one discussion thereof.
    – Isaac Moses
    Feb 3, 2014 at 20:47
  • On a more basic level as a bachur (foolish that I was) I received a psak from 2 Dayanim in Jerusalem that it was forbidden to smoke on Yom Tov since in our day and age it's known how harmful smoking is and the trend is to quit thus not making it "Shave L'Kol Nefesh".
    – eramm
    Feb 4, 2014 at 10:11

4 Answers 4


In the translation of Michtav M'elyahu (Strive for Truth vol. 4 p 355), R' Carmell quotes R' Dessler that halacha does not change even if the reason given for the law seems to be untrue. He says that there may be other reasons other than the one given for the said halacha, and only the most obvious reason was the one stated, so the halacha stands without the fact associated with it.

R' Moshe Feinstein (Igros Moshe - Choshen Mishpat 2:73) says that the halacha depends on the situation at the time of the original ruling of the Sages, whether science (or our understanding of it) has changed. R' Y.B. Soleveitchik (in a recorded discourse) said a similar idea about the chazaka of "tav l'meitav ten du" - that a woman wants to get married - regarding the feminist movement and ramifications of women "becoming independent," namely that the reality at the time of Chazal is the binding reality.

However, the Pachad Yitzchok (not R' Hutner - R' Lampronti of the 18th century) concludes that we cannot kill lice on Shabbos, as the allowance to do so was based on the science of their times which Chazal accepted but we have discovered otherwise.

  • Notwithstanding my agreement to the above comments about the example of the sun. Feb 3, 2014 at 20:50
  • I added some links. Can you add a link to the discourse of RYBS that you mention, or at least a more precise citation? Same goes for the Pachad Yitzchok citation. By "translation of Michtave M'elyahu," do you mean Strive for Truth?
    – Isaac Moses
    Feb 3, 2014 at 21:33
  • Wow thanks for picking up my slack. I don't remember what the title of the recording was, but I could probably find out. Feb 3, 2014 at 21:37

Harav Yishak Yosef Shelit"a, our current Rishon Lesion writes in Yalkut Yosef Kitzur Shulhan Aruch that we should never chose what science says over Hazal. There are a few reasons as to why. One of the most accepted answers is "Nishtanu HaTvaim"- nature has changed and therefor Hazal said what they said because what they said was true at their time.

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    "Nature changed," by itself, would seem to be a good reason to make new rulings that apply Halacha to the new laws of nature.
    – Isaac Moses
    Feb 4, 2014 at 1:28
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    Indeed that reason says that currently chazal are not right and science is. In any event, sourcing any of this would be invaluable.
    – Double AA
    Feb 4, 2014 at 2:48
  • Right, @DoubleAA, but it doesn't say we should accept what science says for practical halachic purposes over what chazal do. Anyway, hachamgabriel, can you cite where he says this more precisely?
    – msh210
    Feb 4, 2014 at 2:49
  • @msh210 it doesn't say we shouldn't either...
    – Double AA
    Feb 4, 2014 at 2:49
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    No, no, @DoubleAA, "nishtane hateva" itself doesn't imply one way or the other. But this is an answer: it says Rabbi Yosef said "we should never chose what science says over Hazal".
    – msh210
    Feb 4, 2014 at 2:51

If I remember correctly, I once saw in Kriene D'igresa from the Steipler that we can decide a machlokes rishonim based on archeological findings. The head of the Living Torah Musuem, R. Deutch, once brought a coin that a diver had found on the bottom of the ocean off of France and that was identified as a Tyre coin. R. Deutch asked whether, since there is a machlokes between Rashi and Rambam in regards to the weight, we can decide the machlokes based on weighing this coin. It turned out to be like the view of the Rambam and I think R. Elyashiv exclaimed "well I guess we see the Rambam was right!"

  • This sounds like the Ramban's addition to his commentary after coming to Israel and seeing half-shekel coins (see his peirush to Ki Sisa). Is this story documented? Jun 2, 2014 at 13:34
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    i saw this in Ami Magazine about 2 or 3 years ago. dont remember at all which issue though.. Jun 2, 2014 at 17:35

Torah is the truth, all halachos are divine providence and are eternal, I know halachos can be taken either in leniency or stringency according to situations, through a rav, but I never heard of actually changing halachos.

  • "all halachos are divine providence and are eternal" what does that even mean?
    – Double AA
    Jun 11, 2014 at 0:00
  • OF course we never change "Halocho" - but surely we sometimes adjust how we behave - "practical Halocho" - based on recent discoveries? Jun 11, 2014 at 8:14
  • @DoubleAA in ani maamin we say that we believe the words of our sages are true, as well as we know we are not allowed to change Torah, halachos therefore pertain to every generation, meaning they are eternal and were given for a reason (hashgacha protis)
    – Malka
    Jun 18, 2014 at 15:12
  • @Malka Hashgacha Pratis doesn't follow from given for a reason, nor does that follow from being eternal, nor does that follow from ability to change them, nor does that follow from trusting the sages. Nor is ani maamin a canonical text.
    – Double AA
    Jun 19, 2014 at 3:27

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