Horayot 4B

בעי רב יוסף: אין חרישה בשבת, מהו? מי אמרינן: כיון דקא מודו בכולהו מלתא, כביטול מקצת וקיום מקצת דמי, או דלמא: כיון דקא עקריין ליה לחרישה כל עיקר, כעקירת גוף דמי?

§ Apropos the ruling of a court to nullify a mitzva, Rav Yosef raises a dilemma: If the court issued a ruling that there is no prohibition against plowing on Shabbat, what is the halakha? The Talmud elaborates: Do we say that since the judges agree to the application of the entire matter of prohibitions on Shabbat and their ruling is exclusively with regard to plowing, its status is tantamount to nullification of a portion of the mitzva and fulfillment of a portion of the mitzva? Or perhaps, since the judges are abolishing the principal category of labor of plowing in its entirety, it is tantamount to abolishing the essence of the mitzva of Shabbat.

תא שמע: יש נדה בתורה אבל הבא על שומרת יום כנגד יום פטור. ואמאי? הא עקריין לשומרת יום כנגד יום כל עיקר!

The Talmud suggests: Come and hear resolution of the dilemma from the Mishnah, which gives as an example of a ruling for which the court is liable to bring an offering: There is a prohibition against having sex with a menstruating woman written in the Torah, but one who has sex with a woman who observes a clean day for a day that she experiences a discharge is exempt. And why should they be liable in that case? Didn’t the judges abolish the halakha of a woman who observes a clean day for a day that she experiences a discharge, in its entirety? Apparently, nullification of one category of halakha in a mitzva is not tantamount to abolition of the essence of an entire mitzva.

אמר לך רב יוסף: שומרת יום דקאמרין כדשנין.

The Talmud rejects this: Rav Yosef could say to you: The ruling concerning a woman who observes a clean day for a day that she experiences a discharge is as we stated, as we explained earlier. The reference is not to abolition of the entire halakha of the woman who observes a day for a day, but rather to a case where the judges ruled that the initial stage of sex is permitted.