Bechoros 5:3. In this Mishnah, we will expand on the prohibition of inflicting a blemish on a bechor, a blemish that will render it permanently blemished. This prohibition is from the Torah: the Torah instructs that a bechor must not have a blemish, and Chazal expounded that this applies not only to directly inflicting a blemish but even to causing one. This prohibition is counted as one of the mitzvos, and one who intentionally inflicts a blemish on a bechor - with witnesses and a warning - receives lashes from the Torah.
The scope of the penalty imposed by the Sages:
If someone transgressed and inflicted a blemish, the Sages penalized him so that he does not profit, and tricksters will not be rewarded. The question is the scope of this penalty. The Mishnah states: One who slits the ear of a bechor, meaning he cuts the cartilage of the ear. According to Rabbi Eliezer, since this person inflicted the blemish through a forbidden act, he can never eat the animal based on that blemish. However, when a new blemish develops on the animal on its own, completely unrelated to the act he performed, and it is a permanent blemish - the kohen owner is allowed to slaughter it based on the new blemish. This means: a person does not profit from having inflicted a blemish on his animal, but if another blemish develops on it in another place, it is permitted for slaughter. This is the approach, and this is the practical halachah.
Furthermore: if a kohen father inflicted a blemish on a bechor, and after his death his sons inherited it - they are permitted to eat it immediately, since they were not the ones who performed the invalid act.
The first incident: The Roman official and the ram:
The Mishnah brings two incidents that illustrate these principles. The first incident: there was a male sheep that had grown old, and its long wool dragged on the ground. One might wonder why the Mishnah used this long description instead of using the word "ram" itself. A Roman official saw the ridiculous-looking sheep, covered in excess wool, and asked: "What is the nature of this?" (The question is not about the natural habits of the animal, but rather an inquiry into its character and essence).
His question was twofold: First, he had never seen such a sheep, and what is this strange appearance? Second, what is the explanation for this behavior? After all, wool is a valuable commodity, and why would a person let the wool go to waste and not shear and collect it? It does not make sense.
They answered him: "Bechor hu, ve'eino nishchat ella im ken hayah bo mum" - it is a bechor sheep, and we are not permitted to slaughter it unless a blemish develops on it, and therefore we leave it to graze. That official thought that the blemish had to occur naturally, and therefore he took a dagger and slit the ram's ear. Why did he do so? For no reason at all, just like vandals who deface a beautiful wall to leave their mark. The main point: he had no intention for his act to have any halachic significance.
Meanwhile, the Jewish owner thought: The ear is slit, and it is blemished - can one rely on this blemish and slaughter the animal? "Uva ma'aseh lifnei chachamim vehitiru" - the Sages replied: what does it matter if a wolf bit its ear or if a Roman slit its ear? We have before us a permanent blemish, and it is permitted to slaughter based on it.
The Roman official saw that immediately after he slit the ear, already the next day they slaughtered and ate it, and he said to himself: I will do a favor for these Jews, and perhaps they will repay me in kind. "Vehalach vetzaram bechoros acherim ve'asru" - Since the non-Jew now acted not for his own sake but to benefit the Jews and permit their bechoros for eating, it is as if the Jews did it themselves, and miderabanan one cannot rely on this blemish for slaughter. This principle is very similar to the laws of amirah le'akum, which forbid asking a non-Jew to do work for you on Shabbos.
The second incident: The children and the tails:
The Mishnah brings another incident, in which a similar thing happened under different circumstances: children who were playing in the field, and having nothing else to do, tied the tails of the sheep to one another. The tail of one of the sheep was torn off, and it was a bechor. Now the bechor is missing a tail, and this is a blemish. The question is: since Jewish children did this, can it be slaughtered based on this blemish? The matter was brought before the Sages, and they permitted it: the children had no intention to tear off the tails, and since they did not attach any halachic significance to their actions but were merely playing - it is considered blemished and permitted for slaughter.
The children saw that they had done a good service, and said: We can have a meat dinner every evening if we just get a few more. They went and tied together the tails of additional firstborn animals, wanting meat for their meals. The Sages said: No. Anything done initially to create a blemish constitutes an intention to inflict a blemish. Even though they are children, since the act was done intentionally - benefiting from it is forbidden, and the owner is not permitted to rely on this blemish to slaughter the sheep.
The Mishnah summarizes the rule: Anything done intentionally to cause an injury or inflict a permanent blemish is forbidden, and it does not permit the animal for eating. But if the matter occurred unintentionally, such as the Roman who sliced the ear or the children who tore off the tails - the animal is permitted, and they are allowed to slaughter it based on that blemish.
The Gemara's question on "this is the rule":
The Gemara asks the question that must always be asked: Wherever it says "this is the rule" - even though we saw an exceptional situation in the last Mishnah of the previous chapter - the normal use of this expression is to summarize the principles of the Mishnah, and regarding this the Gemara always asks: What does the rule come to include? After all, we just learned the Mishnah itself.
The Gemara answers: The rule comes to include grama (indirect causation). That is, even someone who does not inflict the blemish through a direct act, but arranges the circumstances and causes a blemish to be inflicted upon the animal - this blemish is also considered an invalid blemish that one may not slaughter based upon. For example, a person who knows that a certain field has a lot of barbed wire spread out, and says: Let the sheep graze in this field, and among the barbed wire fences their ears will be torn and the blemishes will be created on their own. He sends his sheep to graze there, and indeed they return injured on account of the barbed wire. Since he did not do a direct act but caused the matter indirectly, through the principle of grama, this blemish cannot be relied upon and one may not slaughter based upon it. This is the practical halachah, and this is the novel concept that "this is the rule" comes to include: One who causes a blemish indirectly through the principle of grama.