Our Gemara on Amud Aleph tells us:

Rav said: Adda the fisherman told me that a fish that has sat for some time and is close to spoiling is at its best.

Tosafos here objects, as many of us might, fish close to spoiling is dangerous to eat and far from at its best! Tosafos answers, perhaps this aspect of nature has changed, as we see that many of the medicines discussed in the Gemara are also no longer effective.

The idea that certain physiological processes as understood in the time of the Gemara has changed comes up in halacha from time to time. For example, even though there is a Talmudic tradition that mixing fish and meat together causes leprosy (Pesachim 76b), and we still honor that tradition in halakhic practice, poskim take into account that it does not appear to be dangerous anymore and employ leniencies in certain cases (See Magen Avraham OC 174:6.)

Notably the Rambam skips the entire discussion of fish and meat. Chasam Sofer (Responsum YD 101) suggests, “The Rambam, with his great powers and knowledge of medicine was able to discern that this no longer was a danger.”

While halakhically we are careful to honor traditions, and no modern posek would declare on his own authority that nature has changed and apply it like the Rambam did, we still should keep in mind that it is merely jurisprudence. It is important to know that theologically it can be true so we don’t have to feel like fools denying our common sense. That is to say, we can honor a tradition out of piety and respect but we don’t have to surrender our intellect.  If it seems fishy, indeed it may be so.

I conclude with this thought. If it is acceptable to say that certain natural and physical realities changed since the time of the Talmud, surely this is so for psychological and social realities!  After all, physical nature is far more slow to change and less mutable. 

Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation

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