Resources for Kesubos daf 18
1. The גמרא says that the חכמים say that if ראובן says to שמעון that I owed your father $100 but I paid him $50 that ראובן is not obligated to swear a שבועה דאורייתא since he is a משיב אבידה in the sense that he could have never mentioned the entire loan and then wouldn’t have been חייב anything. רש"י in ד"ה ור"א says that the fact that a משיב אבדה doesn’t need to swear is a תקנת חכמים. תוספות in ד"ה ורבי disagrees and says that משיב אבדה is a פּטור דאורייתא because of a מיגו that he could have remained silent. The פּני יהושע explains that רש"י couldn’t say like תוספות because רש"י held like theרמב"ם in הלכות שלוחין פּרק ט הל׳ ד who says that we don’t say a מיגו לאפטורי בשבועה. (It is fairly common that רש"י and the רמב"ם share the same שיטה). He also quotes this פּשט from his grandfather the מגיני שלמה. רבי עקיבא איגר ז"ל asks that even without משיב אבדה, ראובן should not have to swear מדאורייתא because we have a principle of אין נשבעין על טענת שמא. Therefore, since the son obviously has no knowledge of the loan since he never asked for it, שמעון’s claim that ראובן should swear is only a שמא. The only other possibility is that once ראובן admitted to a loan of $100, שמעון became a ברי because he knows ראובן never paid his father a dime. However, even if this were true, we have a rule that if the הודאה comes prior to the תביעה there is no חיוב שבועה של מודה במקצת. The קובץ שיעורים here brings theרמב"ן in גיטין that changes the story a little. He says the גמרא means that ראובן says I borrowed $100 from your father. שמעון then says I know for a fact you didn’t pay my father back a dime back. ראובן then says that he did pay him $50. In that case the הודאה came before the טענה even though it is a case of משיב אבדה.
2. The משנה says that a person is believed to say they were פּסולי עדות when they signed a certain document if they have a פּה שאסר. רש"י in ד"ה פּסולי עדות says that the פּסול עדות that the משנה is referring to is either the פּסול of being a relative or a משחק בקוביא. The רמב"ן asks that admitting you were a gambler is making yourself into a רשע. If so, how can you be believed to do that—we have a principle אין אדם משים עצמו רשע! (One possibility is that רש"י was only going according to the הוה אמינא of the גמרא where a פּה שאסר would even override the concern of אין אדם משים עצמו רשע. Nonetheless, the רמב"ן does not give that answer. Presumably he considered that a דוחק.) Rather, the רמב"ן answers that רש"י must hold that since משחק בקוביא is only an איסור דרבנן of אינו עוסק בישובו של עולם it isn’t considered making yourself into a רשע. It is not clear if the רמב"ן in focusing on the fact that it’s a פּסול דרבנן or the fact that the issue is only אינו עוסק בישובו של עולם. Nonetheless, the simple reading of the רמב"ן is that being anאיסור דרבנן is not a היתר on its own. The reason is that the רמב"ן is referring to a גמרא in סנהדרין דף כ"ד ע"ב where the גמרא says a gambler is פּסול להעיד מדרבנן. However, the גמרא brings a מחלוקת as to what the פּסול דרבנן is. One opinion is that a gambler violates גזלה דרבנן. The other opinion is that its just אינו עוסק בישובו של עולם. The fact that the רמב"ן feels the need to quote the opinion of אינו עוסק בישובו של עולם makes it sound like the היתר is because אינו עוסק בישובו של עולם is not considered making yourself into a רשע like גזלה דרבנן would be. פּני יהושע says (on his own) that רש"י must hold like the מ"ד that the only issue is אינו עוסק בישובו של עולם.
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