The third perek of Shevuot discusses the shevuat bitui. This refers to an oath regarding the performance of an action, either in the future or past. We learn that if one inadvertently breaks the oath then they are obligated to bring a korban oleh ve'yored (as discussed last week). The first Mishnah records the debate regarding one that made a shevuah not to eat something. R' Akiva maintains that if the person then ate, even the smallest amount, they would have violated the shevuah. The Chachamim however argue that only if the person had a kezayit sized amount they would be liable for breaking the shevuah. The Chachamim reason that in general, one is only liable for consuming a forbidden food if they consumed a kezayit amount. R' Akiva however responds that the area of shevuot is novel and no comparisons can be drawn from other areas of halacha.
The Rambam (Shevuot 4:1) rules like the Chachamim that if after making such a vow, on ate less than a kezayit they would be exempt. The Rambam however compares this case to one that less than a kezayit of non-kosher meat. The comparison implies that while one would not be obligated to bring a korban, it would nevertheless be forbidden to consume less than a kezayit.
The Avi Ezri cites two comments of the Ran. In the first, from his chidushim, the Ran questions the Rambam's position. He explains that chatzi shiur (half the measure that would constitute a violation of the prohibition) is prohibited for a biblical prohibition either based on a pasuk ("kol chelev") and logically based on the concerned that one might continue to eat and reach the prohibited amount. He argues however that for a prohibition that one creates for themselves (by way of shevua) if his declaration applied to a chatzi shiur, then consuming that amount should be liable to lashes. If however "eating" implies only a kezayit's worth, then less than that amount is by definition permitted. In the second comment of the Ran, in his glosses to the Rif, the Ran defend the Rambam explaining that since a chatzi shiur is prohibited because one might add to it until reaches the prohibited amount, that same logic would also apply to shevuot. Who do we understand these two explanations of the Ran.
The Avi Ezri explains that according to the first understanding, chatzi shiur is itself a forbidden since it is considered part of a forbidden object. Consequently, even if it was not possible for the full shiur to be completed, it would still be forbidden to consume the half shiur. The example would be if one ate a half a kezayit of chametz just before the end of Pesach. The Avi Ezri cites R' Akiva Eiger that disagrees with his position. Viewing chatzi shiur in this light, the logic would not stand for shevuot, since what he is eating at that moment is completely permitted – it is not considered part of a forbidden object. The fact that one may consume more is not a basis to forbid the chatzi shiur now.
According to the second understanding however, even if the half shiur is considered a permitted object, since one might increase it to a full shiur, one is not allowed to consume it. When we derive chatzi shiur from the verse "kol chelev", it is not to be understood that the prohibition of chelev applies to the chatzi shiur. Rather a new prohibition of "chatzi shiur" is being derived from the pasuk. Similarly, this prohibition of chatzi shiur applies to anything which would be prohibited if its quantity was increased. This then explains the position of the Rambam, since this understanding of the prohibition of "chatzi shiur" could equally to apply to our case as well. According to this understanding, in a case where it would not be possible for the quantity to be increased, then one could argue that the prohibition of chatzi shiur would not apply.
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