תלמוד לומר (ויקרא יד, יג) 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 leper’s guilt-offering requires placement of the blood and sacrificial parts on the altar.
ואי לא אהדריה קרא ה"א למאי דנפק נפק ולמאי דלא נפק לא נפק ה"נ ה"א אשת אח דאישתרא אישתראי שאר עריות לא
The Gemara 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 relations are forbidden were not permitted at all. Consequently, there is no proof from here that one might have thought that women with whom relations are forbidden are in fact permitted in levirate marriage.
אלא סד"א תיתי במה מצינו מאשת אח מה אשת אח מייבמה אף אחות אשה תתייבם
§ Rather, the suggestion that other women with whom relations 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 relations 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.
מי דמי התם חד איסורא הכא תרי איסורי מהו דתימא הואיל ואישתרי אישתרי
The Gemara wonders about this: Is it comparable? How can one case be derived from the other? There, in the case of a brother’s wife, only one prohibition has been permitted, the prohibition with regard to a brother’s wife, whereas here, we are dealing with two prohibitions, both a brother’s wife and a wife’s sister. The Gemara answers: It is nevertheless necessary to refute this suggestion, lest you say: Since it is permitted, it is permitted. In other words, as the Torah permitted a brother’s wife in levirate marriage despite the fact that she is ordinarily forbidden, she remains permitted even if the additional prohibition with regard to a wife’s sister applies to her.
ומנא תימרא דאמרינן הואיל ואישתרי אישתרי דתניא מצורע שחל שמיני שלו בערב הפסח וראה קרי בו ביום וטבל אמרו חכמים אע"פ שאין טבול יום אחר נכנס זה נכנס
And from where do you say that we state this reasoning of: Since it is permitted, it is permitted? As it is taught in a baraita: With regard to a leper whose eighth day, on which he becomes ritually pure from his leprosy and brings his last offerings to the Temple, occurs on the eve of Passover, and he experienced a seminal emission on that eighth day and then immersed in a ritual bath, the Sages said: Although any other individual who immersed himself that day for purification from his ritual impurity may not enter the Temple before sunset, this leper, who saw an emission of semen and immersed, may enter the Temple.
מוטב שיבא עשה שיש בו כרת וידחה עשה שאין בו כרת ואמר ר' יוחנן דבר תורה אפי' עשה לית ביה
The baraita explains the reason for this exception. It is better that a positive mitzva that includes karet, i.e., bringing the Paschal lamb at the right time, comes and overrides a positive mitzva that does not include karet, i.e., not entering the Temple in a state of ritual impurity. If the leper does not become purified of his leprosy he may not sacrifice the Paschal lamb. And Rabbi Yoḥanan said: By Torah law there is not even the overriding of a positive mitzva in this case of one who immersed himself during the day entering the Temple.
שנאמר (דברי הימים ב כ, ה) ויעמוד יהושפט בקהל יהודה לפני החצר החדשה מאי חצר החדשה אמר (ר' יוחנן) שחדשו בה דברים ואמרו טבול יום לא יכנס למחנה לויה
Rabbi Yoḥanan explains his claim: As it is stated: “And Jehoshaphat stood in the congregation of Judah and Jerusalem, in the House of the Lord, before the new courtyard” (II Chronicles 20:5). What is the meaning of “the new courtyard”? Rabbi Yoḥanan said: This is referring to the place where they issued new matters and said that one who immersed himself that day may not enter the camp of the Levites, which in Jerusalem is the Temple Mount, despite the fact that by Torah law no such prohibition applies.
ואמר עולא מה טעם הואיל והותר לצרעתו הותר לקרויו מי דמי לדעולא
And with regard to this halakha itself, that a leper who experienced a seminal emission may nevertheless sacrifice offerings in the Temple, Ulla said: What is the reason that this is permitted to him? Since it is permitted for his leprosy, i.e., the Torah allowed him to enter the Temple Mount while still a leper to achieve full ritual purification, which requires that he sacrifice offerings, it is permitted with regard to his seminal emission as well. This shows that the tanna accepts the principle that as one prohibition is permitted, two prohibitions are likewise permitted. The Gemara rejects this argument: Is this assumption comparable to the case of Ulla?