http://www.jlaw.com/Commentary/animalt.html.
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Actually, it is not only for taking a human life that an animal is killed. The Torah also rules that if a man or woman engage in bestial relations with an animal, they are both stoned to death. The Mishnah (Sanhedrin 54a) raises our question: "If the person has sinned, how has the animal sinned?" It is interesting to note that the Jews explicitly rejected the idea that an animal can be morally accountable for its actions, including sexual sins, over two thousand years ago, while centuries later, the non-Jewish world was still confused over the issue. From the fourteenth through to the eighteenth centuries, both people and animals involved in bestiality were not merely executed; they were tortured and burned alive. In France, a person who had relations with a Jew suffered the same fate, as a Jew was considered to be no superior to an animal.
Going back many centuries to the Mishnah, the "inferior" Jews were explaining why an animal used for bestiality, although not morally accountable, is nevertheless executed: "Since the person came to harm due to it, the Torah said that it should be stoned. Another explanation: So that the animal should not pass through the streets and people say, 'This is the animal on account of which so-and-so was stoned.' "
The latter explanation accounts for why the animal is removed from the picture; after all, unlike a goring ox, this animal poses no threat to society. But the first explanation seeks to account for the whole business of the stoning. Yet it is somewhat cryptic: "Since the person came to harm due to it, the Torah said that it should be stoned." What does this mean?
If we return to the lethal animals, and to the words of the Ramban, we seem to find the key to the whole topic. Ramban's explanation occurs in the context of God's decrees to Noach regarding the new order in the post-Flood era: "I shall demand your blood, your lives; I shall demand it from all the animals" (Bereishis 9:5). On this verse, Ramban raises our question: "I wonder: If this 'seeking' is in the simple sense of the term, that He shall punish animals for this in the same way as humans — surely an animal does not possess an intellect, that it should be punished or receive reward!" Ramban proceeds to give an explanation that is very similar to the Mishnah's explanation for the stoning of the animal used for bestiality: "But perhaps such is the case only for the blood of man, that any animal which kills him should itself be killed, as a decree of the King. This is the reason behind, 'The ox shall be stoned, and its flesh shall not be eaten' (Shemos 21:28)."
Ramban is noting that there is no general concept of reward and punishment for animals, for there is no moral accountability on their part. Rather, their trial and execution is demanded "only for the blood of man," and for bestiality. The point in these cases is not that the animal is being held accountable for its actions. The trial and execution is not based on the animal at all. Rather, it is based on the human participant. The procedure is designed, as a decree of the King, to uphold the dignity of His servant, man.
Upholding the dignity of man – something that had been lost in the pre-Flood morass of immorality. Upholding the dignity of man – something that necessitated the post-Flood generation being permitted to eat meat, in order to establish their position in the world, higher than that of animals. Upholding the dignity of man – something that requires animals who have negated this to be tried and executed in the same manner as human offenders.
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