Bechoros Daf 17 בכורות דַף 17

Rav Oshaya of נהרדעא quoted a Baraisa which states: רחל בת עז – regarding a sheep the offspring of a goat, or a goat the offspring of a sheep, Rebbe Meir says it is חייב, and the Chochomim say it is exempt. The Gemara eventually offers several interpretations: (1) The case is רחל בת רחל בת עז – a sheep, the offspring of a sheep, which is itself the offspring of a goat. Meaning, its mother was a "נדמה" – [animal which] resembles a different species from its mother. Rebbe Meir holds זיל בתר אימיה – go according to its mother, so this “grandchild,” which resembles its own mother, is not a נדמה, and is subject to bechor status. The Chochomim hold זיל בתר אימיה דאימיה – go after its mother’s mother, which is a different species, so this animal is a נדמה and exempt. (2) The case is רחל בת עז בת רחל – a sheep, the offspring of a goat, which is itself the offspring of a sheep. Rebbe Meir says חזרה שייות למקומן – the sheep status returned to its place, i.e., since the “grandson” resembles its true species, it is not considered a נדמה. The Rabbonon say it does not return to sheep status (since it differs from its own mother). Rav Ashi offers a third interpretation.

The next Mishnah states that if a female sheep which had not previously given birth gave birth to two male lambs, ויצאו שני ראשיהן כאחת – and their two heads came out simultaneously, Rebe Yose HaGlili says both are given to the Kohen, because the plural of הזכרים לה' – the “males” are to Hashem teaches that two animals born simultaneously are both bechoros. The Chochomim say: אי אפשר לצמצם – it is impossible to be exact that both animals should be born at precisely the same moment. Rather, one belongs to the owner, and one to the Kohen. According to this opinion, Rebbe Tarfon says הכהן בורר לו את היפה – the Kohen chooses for himself the better lamb of the two, while Rebbe Akiva says משמנין ביניהן – we rule that the “fat” is between them, meaning the Kohen takes the lesser of the two sheep. [This machlokes is explained on the next Daf.] The second sheep (kept by the owner) must graze until it develops a מום (Rashi discusses the halachah of the first sheep).

Those in Rebbe Yannai’s yeshivah noted that Rebbe Yose HaGlili clearly holds אפשר לצמצם בידי שמים – it is possible to be exact even in matters in Heaven’s hands, i.e., incidentally, such as the simultaneous birth of two lambs. Certainly, he would hold one can be exact בידי אדם – in matters in human hands, i.e., when one intends to be precise, such as in measurements. The Rabbonon hold אי אפשר לצמצם in our Mishnah’s case, which is בידי שמים. Do they hold it is possible to be absolutely precise בידי אדם? After several attempted proofs are deflected, the Gemara quotes a Mishnah about a murdered person whose body was discovered מכוון בין שתי עיירות – precisely between two towns. On the next Daf, the Chochomim are quoted as ruling that the two cities bring one עגלה ערופה together and stipulate that the nearer town should acquire the full calf and fulfill its obligation with it. Since they bring one calf and not two (i.e., one each), we see that it could not be precisely equidistant from both towns, because the Rabbonon hold אי אפשר לצמצם even in the case of בידי אדם.