Our Daf deals with the question when a nursing woman is permitted to eat on Yom Kippur.
Unlike a pregnant woman , one who is in the birthing process or immediate post-birth process, a woman who has recently given birth and/or is nursing is considered to be in immediate danger and is not always exempted from fasting.
In this regard, Rav Yehuda quotes Shmuel who says that as long as the womb is still considered “open”, we insist that she eats whether she says that she needs to or not. Thereafter, whether she says that she needs to eat or not, we do not feed her, the assumption being that it is no longer dangerous for her to fast.
The Gemara notes that this is only the version of his ruling that Rav Ashi taught.  However, Mar Zutra had a different version of this ruling, whereby even after the womb has closed, we feed her for as long as she says that she needs to eat.
Ravina asked Mereimar which version of the ruling is to be accepted, and he told him that we follow the lenient ruling of Mar Zutra, seeing as ספק נפשות להקיל  (we are lenient in case of any doubt regarding danger to life.)
At first glance, it might seem from this that Rav Ashi rejects the view that in case of doubt of danger to life, we are stringent, and do not desecrate the shabbos, and that we only desecrate shabbos in a case of certain danger.
However, it is very difficult to accept that this is indeed his view, given the well-known and universally accepted view amongst earlier authorities that in case of any real doubt of danger to life, we are always lenient.
For example, the Mishna (Yoma 83a)  brings the   rule of Rav Matya that if someone might have  been covered by a rockfall, or might still be alive, we desecrate shabbos to free him, because ספק נפשות להקיל and the Gemara concludes that no one disputes this.
In addition, it is clear from the sugya there that even when most experts (including the patient) say that an ill person need not eat on Yom Kippur, we follow 2 who say that he does, and where the patient and doctor disagree, we always follow those who says he needs to eat.
It seems hardly likely that one of the latest and most authoritative Amoraim would rule against all of that.
It thus seems more plausible that Rav Ashi agrees in principle with the rule of ספק נפשות להקיל  but holds that after the womb has closed, there is not even a doubt anymore- we can assume she is not in danger.
 
However, this puts this into the irreputable category of a מחלוקת מציאות  (debate in a factual matter which can easily be researched)- surely both Rav Ashi and Mar Zutra were able to gather enough cases where nursing women were endangered by fasting to either both see at least some doubt or both agree that there is no real concern?
The next step in the yeshiva-style lomdus (analysis) would usually be to show how the argument is not about facts but about how the halacha relates to the facts.
We could suggest that both Rav Ashi and Mar Zutra agree that there is a small danger involved at this stage but differ as to whether this degree of danger is indeed considered a valid ספק halachically.
At the end of the day, there is always some small risk to anyone who fasts, yet nobody suggests that no one should ever fast because of this concern- it is clear from the fact that the Torah requires a regular person to fast that such risks are not only acceptable, but are meant to be taken for the sake of the Mitzva of fasting.
We have also seen the view of Rav Huna (Shabbos 121a) who disagrees with his son, Rabbah bar Rav Huna’s disapproval of those who are “pious” and do not go round killing snakes and scorpions on shabbos, because the risk is small and there is no end to how much time we can spend killing wasps and the like on shabbos.
As such, we are forced to conclude that there is a line somewhere between what is considered a reasonable though doubtful concern for saving life, for which we certainly desecrate Shabbos and other Mitzvos , and far off and never-ending concerns which are not sufficient reason to justify doing so- After all, if we took all far-fetched concerns into account, it would lead to a situation where shabbos in constantly being desecrated out of paranoia.
It is in this grey area between reasonable concern and exaggerated concern that there is room for debate- everyone has to draw the line somewhere, and just as Rabbah bar Rav Huna and Rav Huna drew it in different places, so do Rav Ashi and Mar Zutra.
Although  the criteria are not necessarily the same,  this trade-off has relevance to another common question, namely what level of risk is one permitted to take in the course of normal living? – one of the main springboards for this question is the sugya of דשו בו רבים  on the other side of our daf, which I hope to have time to address in the future, Hashem willing.
It is also possible that given that the level of risk is in a grey area, even medical experts might have different views on the subject, and different surveys or other sets of evidence could lead one to different conclusions, something we have seen so much lately during the endless debates amongst experts in medicine, virology, epidemiology, statistics, and pretty much everything else.
As such, there is also room to say that Rav Ashi and Rav Zutra do indeed differ enough regarding the facts, and not just regarding where statistically the halachik red line is drawn.
Many of us, myself included, are often frustrated or annoyed at the amount of seemingly ridiculous halachik questions going around regarding pikuach nefesh- after all, we have always been taught this rule of ספק נפשות להקיל – in other words, if you have any doubt, just assume its pikuach nefesh and act accordingly.
Although in situations when one does not the time to ask questions and wait for answers before acting, this dictum remains the golden rule, we have seen from the above that there is indeed a grey area or very fine line between real concerns and halachically insignificant ones, and there is thus still certainly room for some give and take on the subject, when, and only when, there is no immediate urgency to act- תורה היא וללמוד אני צריך!