I have heard many times the principle of "nishtanu hatevayim" applied to things that have medically or physically have changed from the time of the Talmud many years ago.

does this apply to separating between meat and fish? do Jews keep it as a tradition? if i did not grow up keeping it as a non Jew, when i convert must i keep it after my conversion?

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    It should be noted that we do not generally worry about Talmudic medical beliefs, and this case ought to be no different. Indeed, the eminent Magen Avraham holds that meat and fish can be consumed together. – mevaqesh Jul 11 '16 at 22:59
  • Note that nishtaneh hatevah is just one answer, indeed the most radically fundamentalist answer, for discrepancies between Chazal's beliefs, and reality. For a discussion of the lack of authority of Chazal outside of legal matters, see judaism.stackexchange.com/questions/53349/belief-in-midrashim/…. – mevaqesh Jul 11 '16 at 23:02
  • What about a a bal teshuva who never "kept" this? – Yehoshua May 19 '17 at 13:59

I heard in a Shiur from Rav Yisroel Reisman quoting the Chasam Sofer (Responsa 101) who suggests two answers. In the second answer, The Chasam, Sofer says that since we say Nishtane Hateva (nature has changed and thus things stated in the Talmud that once did affect a persons health) technically there should be no reason to keep meat and fish separate. He says, according to this, that if a pot was used to cook fish, and thus absorbed its taste, one does not need to kasher the pot in order to use it for meat despite the concern for the danger.

The basis for this answer is that the Chasam Sofer draws attention to the fact that the Rambam omits certain dinim of cooking meat and fish together, which the Gemara took to be dangerous. The Chasam Sofer attributes this to the fact that, according to the Rambam, nature had changed. Nevertheless, we still keep these dinim under the category of minhag.

Based off of this, Rav Reisman suggests that a convert need not keep a separation between meat and fish, as the only reason that we keep such a practice, is because of the minhag, which of course, a convert does not have.

For a fuller treatment of this topic, see here.

Please note this site is not a replacement for calling a competent Halachic Authority

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    exactly what i was looking for – user9410 May 13 '15 at 12:51
  • @Mefaresh the Chassam Soffer brings the Rambam(or lack thereof), explains that he thinks the real reason for the omission nishtana hateva and writes 'while we should not rely on this practically to eat them together for perhaps this is akin to something prohibited biminyan, and possibly the answers given about giluy would not apply, and therefore we don't eat them, and minhag avoseinu Torah'. – user6591 May 13 '15 at 13:24
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    @Mefaresh cont. Imho he is not saying the reason not to eat it is just a minhag, he is saying the laws of psak might not let us change the law, and the minhag proves this point. As such, this unchanged law would apply to a convert to the religion as well. But I already +1'd as this addressed the question precisely. – user6591 May 13 '15 at 13:24
  • @user6591 im just quoting rav reisman – Shoel U'Meishiv May 13 '15 at 13:24
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    K. Hi. Me again. Bugging you about the opinion in your link. Rabbi Leiberman says the Majority of Rabbis rule not like the Magen Avraham based on the Yad Yehuda quoting a a few tshuvos not like him. Granted the Chavos Yair and co. in the Yad Yehuda number more than a single Magen Avraham, but he completely ignored the fact that the two most widely accepted poskim achronim the Mishna Berurah and Aruch Hashulchan rule like the M.A.! – user6591 May 13 '15 at 20:33

In Shulchan Orach Chaim siman 173 where this law is discussed the Magen Avraham #1 writes "perhaps nowadays there is not such a health issue, as we see many items mentioned in the gemara as dangerous for (i.e. makes one susceptible to) an evil spirit And other issues, and nowadays they cause no harm for the natures changed. Also all these issues follow the nature of the lands. And see Yoreh Deah siman 316 siff 3 and Even HaEzer siman 156. And the Bach writes the same in the name of the Rambam."

This Magen Avraham us quoted concisely in Mishna Berurah #3. The Aruch HaShulchan also brings this Magen Avraham and goes on to mention a Tosafos that writes similarly that some communities won't eat fish after meat while some do.

Most people I know will not mix the two, but are also not aware of all of the above.

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    +1 on the sources, how does this answer the question though regarding a convert? – Shoel U'Meishiv May 13 '15 at 12:34
  • @Mefaresh my point was myth busting the need for natural born Jews to keep to this. I was not aware of your Chassam Soffer calling it a minhag. (Seems strange to call a health issue a minhag. For generations people carried handkerchiefs, does that mean we should? How about blood letting? With leaches?) But I'll check it out Bli Nedder. – user6591 May 13 '15 at 12:44

As discussed previously, fish-and-meat is an odd one that made it onto the halacha books. But it's become normative Orthodox practice in [virtually?] all communities at this point. (Rabbi Hershel Schachter said "well theoretically it should be allowed ... but my wife won't let me serve it at my house ... if I was a guest at someone else's house ... well I should be allowed to eat it ... but ...)

As a matter of practicality, I would advise converts to conform (within reason) to the norms of their local Jewish communities; it's good for communal cohesion. Fish-and-meat (the way it's commonly observed -- no need for separate dishes or anything) isn't that hard to keep, so it's worthwhile for sociological reasons, if nothing else.

(I would, however, add that mainstream centrist Orthodox practice is that it ceases to be a problem in large dilutions; thus you can serve orange-juice-with-Omega-3 marked as "OK-Fish" at a meat meal, though some Hassidim won't eat it.)

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    i appreciate the helpful tips, but this doesnt answer my question. where is Rabbi Schecter basing his opinon on? – user9410 May 13 '15 at 12:58
  • @MOKAY see the linked answer -- the Magen Avraham writes that it's odd that other quasi-medicinal practices fell away, but this one stayed on the books. I read that as "gee that's odd but we keep it anyways", others (including Rabbi Schachter) read that as "the Magen Avraham allows it." – Shalom May 13 '15 at 13:08

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