Resources for Yevamos 105
1. The גמרא says that אביי live longer than רבה since אביי did more גמילות חסדים. In theספר in אהבת חסד in חלק ג פּרק ח בהגה, the חפץ חיים asks ממה נפשך: if there was חסד to be done that couldn’t be done by others, didn’t רבה agree that you must stop learning for a מצוה שאי אפשר לעשות ע"י אחירים? And if it could be done by others, why did אביי stop learning? He answers that it must be that there were others who could do it, but not as good as אביי. In that case according to אביי you may not continue learning as you can help the cause more than anyone, whereas רבה would say don’t stop learning since the חסד can still be performed by others.
2. The גמרא says that a woman who spits blood in front of her יבם is still יוצא since there is surely some actual spit mixed in there. There is a fascinating תוספות רי"ד and מרדכי at the end of חולין who ask how it helps that there is a משהו of spit in the blood—don’t we have a principle of ביטול? If so, the spit is as if it isn’t there! The תוספות רי"ד answers that the concept of ביטול ברוב requires there to be some minimum שיעור. In other words, ביטול doesn’t make it that the spit doesn’t exist at all. Rather, it makes it that it’s חשיבות is gone. Therefore, if there is no שיעור then any amount is still חשוב and ביטול wont work. The מרדכי suggests in his second answer that we only say ביטול when there was some point that the two items that are currently mixed together were separate. However, if from the beginning of their existence they were always mixed together then you cannot consider that ביטול. Besides being a big חידוש, he also seems to say the exact opposite in שבת סימן רנ"ט. There he says that if you squeezed out grapes before שבת you can drink the juice on שבת even though more juice undoubtedly came out on שבת which one cannot drink on שבת. The reason you could drink it is because we can assume that whatever juice came out on שבת is בטל in the rest of the juice. He adds that we don’t even tell him to wait till after שבת even though it is a דבר שיש לו מתירין because anything that came in to existence as a mixture is always ביטול. That seems to be the polar opposite of what he said in חולין. The מנחת חינוך in מצוה ג אות א answers the סתירה by saying that if only one item in the mixture came into existence while mixed then we can say its בטל, but if both items were “born” mixed, then we never say ביטול.
3. The גמרא says that according to ר׳ יוסי a child who is around 6 or 7 (עונת הפּעוטות) can do חליצה. That is a case of a דין דאורייתא where a child of that age is considered somewhat knowledgeable and mature. However, when it comes to מקח וממכר, we say that a child that age can only make a purchase because of a תקנת חכמים. The קובץ הערות in סימן ס"ו אות ו explains the יסוד: when a child is less than 6 or 7, they are the same as a שוטה. However, once they are age 6 or 7, they are considered to have sufficient דעת as evidenced by the fact that the חכמים gave them the power to make purchases at that age. Nonetheless, it doesn’t work מדאורייתא is because there is a גזרת הכתוב that says that anything a child does isn’t valid. However, anytime that גזרת הכתוב is absent, we go back to the default that the child is considered to have sufficient דעת. Therefore, by חליצה, since according to ר׳ יוסי there is no גזרת הכתוב saying a child can’t do חליצה, then any child 6 or above would be considered to have enough knowledge to perform that act and for it to count.
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