Resources for Kesubos 24
1. The משנה says that if one person testifies that another person is a כהן they are believed, and the ברייתא clarifies that they are believed regarding תרומה. The simple reading of the גמרא is that we are even referring to תרומה דאורייתא. The ר"ן on י׳ ע"ב ד"ה גרסינן עלה explains that the reason he is believed is because of עד אחד נאמן באיסורין. He therefore asks on the רמב"ם in הלכות איסורי ביאה פּרק כ׳ הל׳ א-ב who says that an עד אחד is not believed for תרומה דאורייתא but only for תרומה דרבנן (which the מגיד משנה points out means our גמרא was only referring to תרומה דרבנן). Why would an עד אחד not be נאמן for an איסור דאורייתא? The פּני יהושע has a very interesting חשבון. He explains that this is really a case of an עד אחד against a רוב, since most people aren’t כהנים. Therefore, since רוב beats a חזקה and an עד אחד isn’t believed even against a חזקה, then עד אחד won’t be believed against a רוב either. The שב שמעתתא inשמעתתא ו אות ט"ז says that it is really a case of עד אחד against a חזקה since even an unborn כהן in his mother’s womb (assuming she is a ישראלית) is considered a זר. Therefore, he has a חזקת זר until proven otherwise and an עד אחד is not sufficient proof against a חזקה.
2. The גמרא says that there is an איסור עשה for a non- כהן to do ברכת כהנים. רש"י explains that the source is the פּסוק that is speaking to the בני אהרן and tells them כה תברכו which means there is an איסור עשה for anyone else to give the ברכה. There is a גמרא in שבת דף קי"ח that says that רבי יוסי would do whatever his friends would tell him to do, even if they told him to be עולה לדוכן even though he was a ישראל. תוספות there in ד"ה אילו היו says he doesn’t know what איסור there would be in a ישראל doing ברכת כהנים other than a ברכה לבטלה. תוספות seems to have forgotten our גמרא that says there is an איסור עשה?! The רמ"א in דרכי משה in the beginning of סימן קכ"ח answers that there is only an איסור if the non- כהן goes up to duchen by himself, but if he is among other כהנים (and doesn’t make a ברכה) then there is no איסור. The מגן אברהם there in ס"ק א disagrees and says the following answer: ר׳ יוסי happens to hold that a woman is allowed to do סמיכה. That means that when the תורה talks to one group (in that case men) and tells them to do a certain מצוה, it is not forbidding the other group (in that case women) from performing the מצוה but rather just not obligating them. Therefore, when the תורה says כהנים should do ברכת כהנים, that just means that other don’t have to but they can. Our גמרא in כתובות which says there is an איסור עשה is going like רבי יהודה who holds מעלין מתרומה ליוחסין, and he also happens to hold women can’t do סמיכה, and all of that is לשיטתו that when the תורה tells one group to do something then the other group isn’t allowed to do it.
It comes out that according to תוספות, at least in רבי יוסי , the איסור עשה is simply a ברכה לבטלה. The חתם סופר here in his חידושים on כתובות takes issue with that since it seems to be incorrect according to all opinions: the רמב"ם holds a ברכה לבטלה is an איסור לאו דאורייתא. תוספות in ר"ה דף ל"ג ע"א ד"ה הא רבי יהודה says that a ברכה לבטלה is only an איסור דרבנן. So an איסור עשה fits no one. He therefore says that תוספות holds that there is typically just an איסור דרבנן because it is still a praise of Hashem, even if it wasn’t necessary. Meaning you can still bless Hashem who made fruit, even if you aren’t eating fruit. However, when you say אשר קדשנו בקדושתו של אהרן you are flat out lying and that is a lack of מורא ה׳ and is therefore a violation of the עשה of את ה׳ אלקיך תירא. Therefore, ר׳ יוסי would go up and duchen if his friends (which were תלמידי חכמים) asked him to since listening to תלמידי חכמים itself is a קיום of את ה׳ אלקיך תירא-- לרבות תלמידי חכמים.
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