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We are beginning siman 9, where the Chayei Adam discusses the concept of performance of a full melacha versus performance of a partial melacha. The Chayei Adam writes that another application of this concept is where two people perform a melacha together. For example, two people holding a pen together and writing two letters. They are not chayav a korban chatas because they are not chayav mideoraysa.
The Chayei Adam writes that similarly, if two people hold onto a candle together while they light another candle, they would not be chayav mideoraysa.
The source for this halacha is that the pasuk says v’im nefesh achas tech’eta ba’asosa. Chazal learn that the chiyuv is only when one person (nefesh achas) performs the entirety of the melacha (ba’asosa means performing it, i.e., the whole thing).
In the parentheses, the Chayei Adam discusses why this halacha is different from the concept of chatzi shiur. The Gemara at the beginning of the eighth perek of Yoma says that Reish Lakish holds chatzi shiur is muttar, but Rav Yochanan holds that chatzi shiur is assur, even if the punishment does not apply until one reaches a larger amount. We pasken that chatzi shiur is assur min hatorah. Thus, for example, if one eats less than a kezayis of neveilah, it is an aveirah and an issur deoraysa, but would not be subject to malkus until eating a kezayis.
The Gemara brings two possible sources for this issur. One is the pasuk kol cheilev, with the word kol coming to include even a chatzi shiur. Another is a sevara, that if eating less than a kezayis is not an aveirah, why would it be an aveirah when one eats small bites that they have eaten a kezayis? Rather, since the small bites are chazi l’itztarufi (the amounts add up together), one can logically conclude that less than a kezayis on its own is assur as well.
Rashi writes explicitly that the concept of chatzi shiur applies to Shabbos as well. There is a famous Teshuvas Chacham Tzvi which states that it does not apply to Shabbos. He learns that the pasuk of kol cheilev is specifically referring to issurim related to food, so it does not apply to the issurim of Shabbos. The Gemara tells us the minimum shiur for each of the melachos, such as a grogeres for kotzair, so anything less is not assur mideoraysa.
Practically, most poskim, including the Mishnah Berurah, follow Rashi and hold that chatzi shiur applies to Shabbos. Thus, performing only part of the melacha is an aveirah deoraysa, even if there is no punishment. The Chayei Adam writes that chatzi shiur is different from the concept of performing a complete melacha, but does not explain why. Be’ezras Hashem, we will discuss a possible explanation in the upcoming shiur.
Summary
- Although one is not chayav until they perform a full melacha, chatzi shiur is assur. We will explain the difference in the upcoming shiur, be’ezras Hashem
