The Gemara answers. This case is comparable only to that other case: If a bird landed on the red heifer it remains valid, since this is not considered to be like bearing a yoke. If a male animal mounted it to mate with it, it is unfit and may not be used for the ritual. The same applies in this case: When the owner moves the heifer into a pen and it threshes, since the owner is uninterested in this action, it is like the case of the bird and the heifer remains valid. The Gemara asks: What is the reason for the difference between the case where a bird lands on it and where a male animal attempts to mate with it?

Rav Pappa said that the verse says: “And the elders of that city shall take a heifer of the herd, which has not been used for work, and which has not drawn in the yoke” (Deuteronomy 21:3). If it were written: “He worked [avad],” and we read: “He worked [avad],” this word choice would indicate that the heifer could still be used until he, the owner of the heifer himself, used it willingly for labor. If it were written: “It was worked [ubbad],” and we read: “It was worked [ubbad],” it would indicate that even if it performed labor on its own it is also prohibited to use it, since some form of labor had been done with it.

Now that it is written: “He worked [avad],” but we read this word as: “It was worked [ubbad],” both meanings are intended: It is prohibited if it is worked in a similar manner to the way he works. In other words, just as the owner works the animal in a way that is beneficial to him, so too, the animal becomes prohibited only when it is worked in a way that causes him to derive benefit from this labor. Therefore, it still may be used if a bird lands on it, because the owner does not derive benefit from this in any way. However, if a male bull mates with this heifer it is rendered unfit, since the owner generally has an interest in this occurring.

The Gemara cites an additional proof. Come and hear from that which is taught: If one finds a lost item, he may not spread it out over a bed or over a frame for his own purposes, since this is deriving benefit from an object that does not belong to him. However, he may spread it out over a bed or a frame for its own sake if it requires airing. If guests happen to come to him, he may not spread it out, neither for its sake nor for his own purpose. Apparently, the benefit is unavoidable and intended, as there is no other way for him to care for the lost object, and he benefits from having his guests see the item; nonetheless, it is still prohibited. This seems to prove that Rava’s opinion is correct.

The Gemara rejects this conclusion: It is different there, as he risks damaging it, either due to the evil eye that he casts upon it or due to the thieves who will now know that this valuable item is in his possession and will attempt to steal it. It is not prohibited because of the benefit; rather, it is prohibited due to the concern that he may damage the item.

The Gemara offers a final proof. Come and hear a proof based on the following mishna: Clothing merchants who sell garments made of diverse kinds, a prohibited mixture of wool and linen, may sell them as they normally would to gentiles. A merchant may place the garments he is selling on his shoulders and need not be concerned about the prohibition against wearing diverse kinds, provided he does not intend to benefit from the garments in the sun as protection from the sun, or in the rain as protection from the rain. However, the modest people, those who are meticulous in their performance of mitzvot, suspend the wool and linen garments on a stick behind them.

And here, isn’t it a case where it is possible for all clothing merchants to act like the modest people and not derive benefit from the mixture of wool and linen? Nonetheless, the mishna states that when one does not intend to benefit from the prohibited item, it is permitted to do so. This presents a conclusive refutation to he who taught the first version of Rava’s statement. According to this version, one is prohibited from deriving benefit when it is possible to avoid doing so and he does not intend to derive benefit. The Gemara concludes: Indeed, it is a conclusive refutation.

It was taught in the mishna that one may not even light the oven with leavened bread. The Sages taught in a baraita: With regard to an oven that one lit with the peels of orla fruit, or with straw of grain that was planted in a prohibited mixture of diverse kinds in a vineyard, if it was a new oven, and by lighting it he hardened the oven and made it stronger for use in the future, then it must be shattered. Since prohibited items were used in the process of forming the oven, one may not derive benefit from the use of the prohibited items. However, if it was an old oven, it may be cooled, and it is prohibited to use the oven only while it is still hot.

With regard to one who baked bread in the oven while it was heated or strengthened by the prohibited items, Rabbi Yehuda HaNasi says: It is prohibited to eat or derive benefit from the bread, since prohibited items were involved in the process of preparation. And the Rabbis say: It is permitted to eat and derive benefit from the bread. If he cooked the bread over the coals that remained from prohibited wood, everyone agrees that it is permitted.

The Gemara asks. Wasn’t it taught in a baraita: Whether it was an old oven or a new oven it may be cooled; there is never a need to shatter the oven? The Gemara answers: This is not difficult. This baraita, which states that one is required to shatter the oven, is in accordance with the opinion of Rabbi Yehuda HaNasi. And that baraita, which states that it is sufficient to let the oven cool, is in accordance with the opinion of the Rabbis. Since the prohibited objects merely strengthen the oven, the Rabbis hold that it is enough to let the oven cool. By cooling the oven one no longer derives benefit from the prohibited items used to light it, and there is no need to shatter the oven.

The Gemara challenges this answer: Say that you heard that Rabbi Yehuda HaNasi prohibits one from deriving benefit from bread baked using the prohibited objects as kindling because there is improvement from the wood used to light the oven in the bread, and therefore, it is prohibited. However, in a different case, namely, when both this and that cause it, i.e., both permitted and prohibited items contribute to the result, such as when one subsequently bakes in this oven and benefit is derived both from the prohibited wood that strengthened the oven and from permitted wood that is used in subsequent baking, did you hear him say that it is prohibited as well? Rather, reject this explanation and say: This is not difficult. This stringent baraita is in accordance with the opinion of Rabbi Eliezer, who says that in a case where both this and that cause it, it is prohibited. And that lenient baraita is in accordance with the Rabbis, who disagree with regard to that principle.

The Gemara asks: Which statement of Rabbi Eliezer serves as the basis for this explanation? If you say that it is the opinion of Rabbi Eliezer with regard to leaven, as we learned in a mishna: In a case where non-sacred leaven and leaven of teruma fell into non-sacred dough, and neither one alone is potent enough to cause the dough to become leavened bread, and they were joined together and caused the dough to become leavened bread, there is a dispute as to whether this dough is considered to be teruma or non-sacred bread. Rabbi Eliezer says: I follow the final element that fell into the dough. If the teruma fell in last, the dough is prohibited to non-priests. And the Rabbis say: Whether the prohibited item, i.e., the teruma, fell in first or the prohibited item fell in last, it never renders the dough prohibited