Yevamot 7B

תלמוד לומר: ״כי כחטאת האשם הוא״. מה חטאת טעונה מתן דמים ואימורים לגבי מזבח ⁠— אף אשם טעון מתן דמים ואימורים לגבי מזבח.

Therefore, the verse states: “As the sin-offering…so is the guilt-offering” (Leviticus 7:13), to teach that just as the sin-offering requires placement of the blood and sacrificial parts on the altar, so too, the metzora’s guilt-offering requires placement of the blood and sacrificial parts on the altar.

ואי לא אהדריה קרא ⁠— הוה אמינא: למאי דנפק ⁠— נפק, ולמאי דלא נפק ⁠— לא נפק. הכא נמי, הוה אמינא: אשת אח דאישתרא[י] ⁠— אישתראי, שאר עריות ⁠— לא.

The Talmud comments: And had the verse not explicitly restored this case of guilt-offering to its generalization, I would say: With regard to that which was excluded from the generalization as a novel ruling in this case, it was excluded, and with regard to that case which was not excluded, it was not excluded, and therefore the halakha would have been different in the various cases. Here, too, I would say: A brother’s wife who was permitted is permitted, whereas the other women with whom sex are forbidden were not permitted at all. Consequently, there is no proof from here that one might have thought that women with whom sex are forbidden are in fact permitted in levirate marriage.

אלא, סלקא דעתך אמינא: תיתי ב״מה מצינו״ מאשת אח. מה אשת אח מייבמה ⁠— אף אחות אשה תתייבם.

§ Rather, the suggestion that other women with whom sex are usually forbidden might be permitted for levirate marriage was based on a different argument: It might enter your mind to say: Let this claim be derived by the hermeneutical principle of: What do we find with regard to, which is a principle of inductive reasoning involving a comparison between cases that include similar details. In other words, the halakha of all other women with whom sex are forbidden can be derived from that of a brother’s wife: Just as a brother’s wife enters levirate marriage, so too, a wife’s sister should enter into levirate marriage.