Resources for Yevamos daf 4
1. Our גמרא says that we learn from the סמיכות of ציצית to כלאים that one can wear ציצית which contain כלאים and from there we learn to כל התורה כולה that עשה דוחה לא תעשה. The רמב"ן in פּרשת יתרו פּרק כ׳ פּסוק ח׳ famously explains that even thoughלא תעשהs are more חמור than עשהs, עשהs are still דוחה since עשהs come from מדת האהבה whereas לא תעשהs come from מדת היראה. What is striking is that if you look in the ראשונים it is not clear that כלאים בציצית is a case of עשה דוחה לא תעשה at all! For example, תוספות quotes ר"ת in חולין דף ק"י ד"ה טלית who says that כלאים is allowed in ציצית by day and night, by men and women, owned and borrowed. We know there is no חיוב ציצית at night, nor by women nor with borrowed garments. So if the היתר of כלאים בציצית was because of דחייה, then it should be an איסור דאורייתא to wear those ציצית at night. It would seem ר"ת holds that כלאים בציצית is הותרה. This is also the opinion of the רמב"ן inשבת דף קל"ב ע"ב ד"ה ודאמרי who says explicitly that כלאים בציצית is not מתורת דחייה but rather because of a היקש and it is הותרה. The רמב"ן here (who seems to be סותר himself from what he wrote in שבת) asks that our סוגיא seems clear that כלאים בציצית works מטעם דחייה since the whole point of the סוגיא is to learn from there ל"ת דוחהעשה . If so, how could anyone suggest it is הותרה? The קובץ הערות in סימן ט׳ answers for שיטת ר"ת by saying that even though by ציצית it is הותרה, we can still learn from there to דחויה since the יסוד of the היתר is the מצוה that exists in the בגד. For example, if the בגד had one of the corners chopped off then even ר"ת would say there is no longer a היתר כלאים. Therefore, we can still learn from there that a מצוה trumps an עברה. However, there is another kind of הותרה like the הותרה of אשת אח ליבום or a כהן being מטמא to relatives where the מצוה can only occur when there is an איסור. In those cases the לאו simply never existed there and one could therefore not learn from that type of case that עשה דוחה ל"ת.
2. The גמרא says that we need the words לא תלבש שעטנז to teach us that שעטנז is only אסור when you have הנאה from wearing it. The בית הלוי in חלק א׳ סימן א׳ אות ה׳ asks that if so, why do we need עשה דוחה ל"ת to allow כלאים בציצית? If כלאים is only אסור when you have הנאה from it, why don’t we apply the principle of מצוות לאו ליהנות נתנו and say there is no איסור here at all? He answers that perhaps we only say מצוות לאו ליהנות נתנו where you must do the מצוה, and we therefore we don’t consider your הנאה to be significant. However, if it is an optional מצוה such as by ציצית where you don’t need to wear a four cornered garment if you don’t want to, perhaps we would say there that you are considered getting הנאה. Furthermore, the only הנאה that would be allowed is the הנאה from the strings which is the מצוה but not the הנאה from the garment itself which is just as אסור as the strings.
3. The גמרא says that רבי יהודה is only דורש סמוכין in משנה תורה since it is מופנה ומוכח. See תוספות ד"ה וכי מפּני who says that the main point is really that it is מופנה or מוכח and we happen to know that all דרשות in משנה תורה are מופנה ומוכח but רבי יהודה
would be willing to be דורש סמוכין even in other ספרים. See the ריטב"א here who disagrees and says that ר׳ יהודה holds that in the rest of the תורה you can’t beדורש סמוכין because of the principle of אין מוקדם ומאוחר בתורה. However, משה who said משנה תורה said it in order. The ריטב"א also says that even the חכמים agree that for עשה דוחה ל"ת you need a דרשה that is מופנה ומוכח because to learn something out that is a בנין אב to כל התורה כולה it must be מופנה ומוכח.
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