Chatzi Shiur on Yom Kippur: Ma'aseh Achilah or Issur Lo Se'uneh?

The shiur opens with a fundamental difficulty posed by the Maran Chov and Tosfos on Yom Kippur: The Gemara (גמרא) discusses whether chatzi shiur (less than the prohibited measure) is forbidden on Yom Kippur according to the dispute between Rav Yochanan and Reish Lakish. Rav Yochanan holds chatzi shiur is prohibited min haTorah, while Reish Lakish holds it is permitted min haTorah (though prohibited derabbanan). The difficulty is stark: how can there be an issur of chatzi shiur on Yom Kippur when the prohibition of Yom Kippur is lo se'uneh (affliction), not a standard issur achilah (eating prohibition)? By other issurim like chelev, we can learn chatzi shiur from the concept that each molecule is prohibited. But on Yom Kippur, the issur is inui nefesh—afflicting the soul—not consuming a prohibited substance. How can we derive chatzi shiur from chelev when the fundamental nature of the prohibition is different? The Rambam (רמב"ם) in Hilchot Ma'achalot Asurot rules like Rav Yochanan that chatzi shiur is prohibited min haTorah. Yet in Hilchot Chametz U'Matzah, the Rambam cites a specific pasuk for chatzi shiur by chametz, and the Kesef Mishne asks: if we already have a general principle of chatzi shiur asur min haTorah, why do we need a special pasuk for chametz? The Mishna LaMelech answers that chatzi shiur only applies to something that was always prohibited, like chelev. But chametz, which was permitted before Pesach (פסח) and will be permitted after, cannot have the same din of chatzi shiur without a specific pasuk. This deepens our question about Yom Kippur: if even chametz needs special justification, certainly Yom Kippur—where the issur is lo se'uneh, not achilah—requires explanation.