Yoma, Mishnah 5. In the previous mishnah, we dealt with the way children begin to fast on Yom Kippur, and the fact that before a certain age they do not fast at all. In the mishnah before us, we will discuss individuals who may find themselves in a situation of possible danger, and who therefore cannot fast.
A Pregnant Woman Who Smelled Food:
The mishnah establishes that regarding a pregnant woman who smelled a certain food, and her craving for it is so intense that medically, if she does not eat from it, a danger to her and the fetus will arise - "we feed her until her soul is restored," until her composure returns and she feels like herself, for this is a matter of pikuach nefesh for both her and her child.
The Gemara teaches that handling this situation is done in stages:
We whisper in her ear that today is Yom Kippur. At times, this whisper has a calming effect, and the craving passes.
If this is not effective, initially - when possible - we feed her in measured amounts (shi'urim): less than the volume of a date for food, and less than a mouthful for liquids.
If it is clear that this method will not suffice, or if there is medical knowledge indicating it will not help - we feed her as much as needed.
An Ill Person - According to Experts:
And what if there are no experts? The mishnah says: "If there are no experts there - we feed him according to himself" - when no medical experts are available, we rely on the patient's own assessment and feed him according to what he deems necessary, until he says it is enough and he requires no more.
"The Heart Knows Its Own Bitterness":
The Gemara explains that the mishnah can be interpreted with an additional leniency: even when experts are present and available who determine that the patient does not truly need to eat, if the patient himself claims against their advice that he does need to eat - it is regarded as if there are no experts present at all, and we listen to the patient. The basis for this is a verse in Mishlei: "The heart knows its own bitterness" - we rely on the person himself and feed him according to his word.
In summary: In this mishnah, we learned of two situations where pikuach nefesh overrides the fast: a pregnant woman who smelled food, who is fed until her soul is restored - after whispering in her ear and initially feeding her in measured amounts, and if that is insufficient, as much as necessary; and an ill person, who is fed based on the assessment of experts, or in their absence according to himself until he says it is enough. And we learned from the Gemara that even in the presence of experts, the patient's own claim is decisive, for "the heart knows its own bitterness."