- Rebbe Eliezer’s source that a metzora shaves with a razor: ראשו
The Gemara asks from where Rebbe Eliezer derives that a metzora must shave with a razor, and answers that he darshens like the following Baraisa: "ראשו" מה תלמוד לומר – What is “his head” written by metzora saying, since it already taught that he must shave all his hair? לפי שנאמר גבי נזיר תער לא יעבור על ראשו – Because it is said regarding a nazir, “A razor shall not pass over his head,” יכול אף נזיר מצורע כן – one could think even a nazir who is a metzora would be similarly forbidden to shave his head, תלמוד לומר "ראשו" – therefore it says, “his head,” that even a nazir who is a metzora must shave his head. Since a nazir’s prohibition is only to shave with a razor (in accordance with Rebbe Yonasan on Daf 39), it would only need to be violated if a metzora must shave with a razor. Otherwise, the Torah would require shaving a metzora nazir with a different implement, because of the principle of Reish Lakish that the Torah only allows for a mitzvah to override a prohibition when there is no possibility to fulfill both.
- According to the Rabbonon, "ראשו" teaches a metzora shaves in violation of הקפת ראש
The Gemara asks what the Rabbonon (who derived that a metzora shaves with a razor from "זקנו") learn from the word ".ראשו" It answers: למידחי לאו דהקפה – To override the prohibition of rounding the head, as a Baraisa relates that "ראשו" teaches that a metzora must shave all his hair, despite the normal prohibition to round the corners of one’s hair. The Gemara asks why this passuk is needed, since we have already learned from זקנו that a metzora shaves in violation of the prohibition to cut one’s beard. The Gemara answers that without the word ראשו, one could think that הקפת כל הראש לא שמה הקפה – rounding the entire head is not considered “rounding” and always permitted (and the prohibition is only when leaving the rest of his hair), so ראשו teaches that the metzora’s shaving his entire head is a (required) violation of הקפת ראש. זקנו is necessary to teach that a metzora must shave with a razor, as the Rabbonon derived it on Daf 39 (that a metzora must violate the prohibition of cutting his beard, which they hold is with a razor).
- Rebbe Eliezer’s source for עשה דוחה לא תעשה is derived from making tzitzis from shaatnez
The Gemara above noted that the Rabbonon can derive the principle that עשה דוחה לא תעשה – a positive commandment overrides a prohibition, from the word ראשו, which taught that a metzora shaves his head despite the normal prohibition to round the corners. The Gemara asks, from where does Rebbe Eliezer, who holds this word teaches a nazir metzora must shave, derive this principle? Tosafos explain that although this too demonstrates that a mitzvah (metzora shaving) overrides a prohibition (of a nazir to shave), the Gemara knows that Rebbe Eliezer has some prior source for this principle, for otherwise he could not take the liberty to apply the passuk to a nazir, who is additionally violating a positive mitzvah to grow his hair. The Gemara answers that he learns it from גדילים (tzitzis), as a Baraisa states: לא תלבש שעטנז – “You shall not wear shaatnez,” and the following passuk commands attaching tzitzis to a garment, הא גדילים תעשה לך מהם – But “twisted cords you shall make for yourself” from them, i.e., you shall make tzitzis of shaatnez.
