Nazir 41A

ותו, אי סלקא דעתך כי עביד במלקט וברהיטני ⁠— מצוה קא עביד, מדלא כתיב ״תער״, כריש לקיש, דאמר:

And furthermore, if it should enter your mind that when he performs his shaving with tweezers or a plane he performs a mitzva, from the fact that the word razor is not written he should not be allowed to use a razor at all, in accordance with the opinion of Reish Lakish, who says a principle in this regard.

כל מקום שאתה מוצא עשה ולא תעשה, אם אתה יכול לקיים את שניהם ⁠— מוטב, ואם לאו ⁠— יבא עשה וידחה את לא תעשה.

The Talmud cites the relevant principle: Any place where you find a positive mitzva and a prohibition that clash with one another, if you can find some way to fulfill both, that is preferable; and if that is not possible, the positive mitzva will come and override the prohibition. In this case the metzora can fulfill his duty with tweezers or a plane, and therefore it should be prohibited for him to shave with a razor. Since it was necessary for the verse to say that the positive mitzva for a metzora to shave overrides the prohibition against destroying one’s beard, this proves that the mitzva can be performed only with a razor.

ורבי אליעזר מאי טעמא? יליף מ״ראשו״. דתניא: ״ראשו״, מה תלמוד לומר? לפי שנאמר גבי נזיר ״תער לא יעבור על ראשו״, יכול אף נזיר מצורע כן ⁠— תלמוד לומר: ״ראשו״.

The Talmud asks: And R' Eliezer, what is his reason? From where does he derive the halakha that a metzora must use a razor? He derives it from the fact that the verse states with regard to a metzora: “He should shave all his hair off his head and his beard” (Leviticus 14:9). As it is taught in a baraita: Why does the verse state: “His head,” if it already stated: “All his hair”? It is because it is stated with regard to a nazirite: “No razor should come upon his head” (Numbers 6:5), and therefore one might have thought that the same should also apply to a nazirite who is a metzora, i.e., it should be prohibited for him to shave his head even for the purification ritual for his tzara'at. Therefore, the verse states: “His head,” to teach that a nazirite who is a metzora must shave his head with a razor. Being that a nazirite is prohibited only to use a razor, if a metzora could fulfill his obligation to remove his hair using other implements, a nazirite who is a metzora would not be permitted to use a razor. Therefore, it can be inferred from the halakha of a nazirite who is a metzora that the only way for a metzora to remove his hair is with a razor.