Daf Yomi - Yoma 74 - Half a Date

In order to be chayav on Yom Kippur for eating, one must eat at least the size of a large date, which will provide him satisfaction. Less than that amount, he will not be satiated and therefore not liable. The Gemora brings an argument between Reb Yochanan and Rish Lakish regarding one who eats less than this amount, if it will be forbidden min haTorah or not.

The Yerushalmi in Terumos disagrees and holds that even though Rish Lakish normally holds that chatzi shiur is permitted, by Yom Kippur he agrees that he will be chayav. The Gaon explains the reasoning. The Torah does not state by Yom Kippur that one should not eat, rather it says that a person should be afflicted, which we learn from there that one shouldn't eat and be satiated. Eating an amount equivalent to a date will satisfy a person fully and eating less than that will satiate him somewhat. This is why Rish Lakish agrees.