As, when a curtain had a worm which made a tear in it, they would tear the curtain further to lengthen the tear, and that enabled them to then sew it in a manner that obscured the tear.

Rav Zutra bar Toviya said that Rav said: One who tightens the thread of a stitch on Shabbat is liable to bring a sin-offering. If two parts of a garment that were sewn together begin to separate, and one pulls the thread to reattach them, it is tantamount to having sewn them. The Gemara cites additional halakhot cited by Rav Zutra in the name of Rav. And one who learns even one matter from a magosh, a Persian priest, is liable to receive the death penalty. And one who knows how to calculate astronomical seasons and the movement of constellations, and does not do so, one may not speak with him because his actions are improper.

The Gemara proceeds to discuss the additional halakhot cited by Rav Zutra bar Toviya. With regard to the magosh, Rav and Shmuel disagreed. One said that they are sorcerers, while the other said they are heretics. The Gemara adds: Conclude that Rav is the one who said that they are heretics, as Rav Zutra bar Toviya said that Rav said: One who learns one matter from the magosh is liable to receive the death penalty. As, if it should enter your mind that they are sorcerers, wasn’t it written: “When you come into the land which the Lord your God gives you, you shall not learn to do after the abominations of those nations. There shall not be found among you any one that makes his son or his daughter to pass through the fire, one that uses divination, a soothsayer, or an enchanter, or a sorcerer” (Deuteronomy 18:9–10)? And the Sages inferred: You shall not learn to do, but you may learn to understand and to teach the topic of sorcery. Apparently, merely learning about sorcery does not violate a prohibition. Only acting upon that learning is prohibited. Rav, who prohibited learning even a single matter from a magosh, must hold that they are heretics, not merely sorcerers. The Gemara states: Indeed, conclude that Rav is the one who said that they are heretics.

Rabbi Shimon ben Pazi said that Rabbi Yehoshua ben Levi said in the name of bar Kappara: Anyone who knows how to calculate astronomical seasons and the movement of constellations and does not do so, the verse says about him: “They do not take notice of the work of God, and they do not see His handiwork” (Isaiah 5:12). And Rabbi Shmuel bar Naḥmani said that Rabbi Yoḥanan said: From where is it derived that there is a mitzva incumbent upon a person to calculate astronomical seasons and the movement of constellations? As it was stated: “And you shall guard and perform, for it is your wisdom and understanding in the eyes of the nations” (Deuteronomy 4:6). What wisdom and understanding is there in the Torah that is in the eyes of the nations, i.e., appreciated and recognized by all? You must say: This is the calculation of astronomical seasons and the movement of constellations, as the calculation of experts is witnessed by all.

We learned in the mishna, among those liable for performing primary categories of labor: One who traps a deer or any other living creature. The Sages taught in a Tosefta: One who traps a ḥilazon and breaks its shell to remove its blood for the dye is liable to bring only one sin-offering. He is not liable for breaking the shell. Rabbi Yehuda says: He is liable to bring two, for performing the prohibited labors of trapping and for threshing, as Rabbi Yehuda would say: The breaking of a ḥilazon is included in the primary category of threshing, as its objective is to extract the matter that he desires from the shell that he does not. The Rabbis said to him: Breaking the shell is not included in the primary category of threshing. Rava said: What is the rationale for the opinion of the Rabbis? They hold: Threshing applies only to produce that grows from the ground. One who extracts other materials from their covering is exempt. The Gemara asks: Even if extracting blood is not considered threshing, let him be liable for taking a life as well. Rabbi Yoḥanan said: This is referring to a case where he broke its shell after it was dead.

Rava said: Even if you say that he broke it when it was alive, he is exempt. Since he had no intention of killing the ḥilazon, he is considered as one who is acting unawares with regard to taking a life. The Gemara raises a difficulty: Didn’t Abaye and Rava both say that Rabbi Shimon, who rules that an unintentional act is permitted, agrees that in a case of: Cut off its head and will it not die, one is liable? One who performs an action that will inevitably result in a prohibited labor cannot claim that he did not intend for his action to lead to that result. Lack of intention is only a valid claim when the result is merely possible, not inevitable. Since one who extracts blood from a ḥilazon inevitably takes its life, how can Rava claim that his action is unintentional? The Gemara answers: Here it is different, as the longer the ḥilazon lives, the better it is for the trapper, so that its dye will become clear. Dye extracted from a live ḥilazon is a higher quality than that which is extracted from a dead one. Rabbi Shimon agrees that one who performs an action with inevitable consequences is liable only in a case where the consequences are not contrary to his interests. Since he prefers that the ḥilazon remain alive as long as possible, he is not liable for the inevitable consequences.

We learned in the mishna, among those liable for performing primary categories of labor: And one who slaughters an animal on Shabbat. The Gemara asks: As there was no slaughter necessary for construction of the Tabernacle, one who slaughters an animal, due to what prohibited labor is he liable? Rav said: He is liable due to dyeing, as in the course of the slaughter the hide is dyed with blood. And Shmuel said: He is liable due to taking a life.