Resources for Yevamos daf 88
1. The גמרא seems to understand that it is obvious that עד אחד is נאמן באיסורים in a case where it isn’t איתחזק איסורא. There are a few ways to understand this. רש"י in ד"ה ואמר ברי says this is based on סברא: if you couldn’t trust anyone even for regular איסורים then how could anyone eat in their friends' homes? תוספות ישנים is unmoved by this and feels that people can just be careful with what they eat. Therefore, תוספות ישנים says it is based on a לימוד from נדה where she is trusted to count her days and say she is טהורה. However, תוספות in ד"ה ברי says the same פּשט as the תוספות ישנים but asks that if we learn from נדה then it should be obvious that an עד אחד is believed even if it is איתחזק איסורא (since a נדה is איתחזק איסורא) and the גמרא is not sure about that? He answers that נדה is different since she isn’t איתחזק to see דם forever. The מהרי"ט quoted in שב שמעתתא ג, פּרק ט says that we can prove from here that a חזקה העשויה להשתנות (a חזקה that by definition won’t continue forever) must not be considered a חזקה at all. However, the שב שמעתתא himself disagrees and says of course it is considered a good חזקה. All תוספות meant was that the עד אחד is not contradicting the חזקה when she says she is טהורה since the חזקה doesn’t say it will always last. However, where no one argues with a חזקה העשויה להשתנות we would say it is still in effect.
2. As mentioned, the גמרא is unsure if an עד אחד is believed even against a חזקת איסור. The קובץ הערות, in סימן ס"ז אות ד discusses what exactly the question is: one would think that the question is simply if עד אחד is stronger than חזקה or not. However, this can’t be true since חזקה can get someone killed (such as where a woman has a חזקת אשת איש and is מזנה) whereas an עד אחד is never believed to kill someone, so how could we think that עד אחד would beat חזקה? He answers that חזקה and עדות work on totally different tracks. חזקה is just a הנהגה of how ב"ד operates and doesn’t tell us what actually happened. By contract, עד אחד is a בירור הדבר and tells what really happened. Therefore, it is possible that where and עד אחד is believed it is more powerful than a חזקה which is just a הנהגה On the other hand, it could be that we don’t listen to an עד אחד when we have a חזקה ודאית. That was the גמרא’s question.
3. The גמרא says that we can’t prove whether an עד אחד is believed to say a woman’s husband is dead from a case of ספק איסור because an עד אחד is not believed when it’s איתחזק איסורא plus a דבר שבערוה. The “plus” is not so clear and is a מחלוקת ראשונים. The רמב"ן says that the question was that even if you would say that an עד אחד is believed against a חזקה, it wouldn’t be believed against דבר שבערוה even if the דבר שבערוה didn’t have a חזקה. The רשב"א here as well as תוספות in גיטין דף ב ע"ב ד"ה הוי sound like they understand that the main question was from איתחזק איסורא and we just strengthened the question by adding that it’s also a דבר שבערוה, The נפקא מינה between these explanations is a case where it is a דבר שבערוה where it wasn’t איתחזק איסורא. While most of us would probably assume that a דבר שבערוה requires two עדים no matter what, the שב שמעתתא in ו׳, ג brings a תשובות מיימוני and מהרי"ק who say that even an עד אחד is believed by a דבר שבערוה if it isn’t איתחזק איסורא. The case the תשובות מיימוני was discussing was where a man sent a שליח to be מקדש a woman for him and the שליח said "הרי את מקודשת לי" instead of "לראובן". He swore it was just a slip of the tongue and the תשובות מיימוני says he is believed and she doesn’t need a גט from him. The שב שמעתתא explains that even though we know there is a principle of אין דבר שבערוה פּחות משנים, that only applies where you know it is a דבר ערוה independent of this עד, but if the עד is saying there never was a דבר שבערוה here then we can believe him.
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