One of the sages, Rav, ruled that one who leaves kosher meat unsupervised on his table may not come back and eat that meat lest it have been switched with meat from an animal which died without ritual slaughter. The scholars asked: The early sage (Tanna) permitted buying from a gentile merchant if it is known that he got kosher meat from a Jewish shochet, meaning that even though the meat was not under supervision we do not fear it has been replaced by meat from an animal which died without ritual slaughter. Answer: The gentile will not switch the kosher meat with meat from an animal which was not slaughtered in accordance with ritual, but meat left on a table may attract crows who will take the kosher meat and leave, in its place, meat which has not been ritually slaughtered. The scholars then asked: The early sages ruled that if one finds a piece of meat in front of stores, the majority of which sell kosher meat and the minority non-kosher, the meat is permitted for eating, for we suppose the meat came from one of the kosher stores. This means that we do not suspect that crows have replaced a piece of meat which has been left on the ground with no supervision and left meat which was not ritually slaughtered in its stead. Answer: This case refers to a piece of meat left in the hands of the gentile, but if it indeed was left on the ground in front of the stores the piece is forbidden for consumption lest crows have switched the kosher meat for non-kosher. The scholars then asked: The early sages ruled that one who finds, in a city, a pot of cooked meat — if most people who would eat cooked meat in that city are Jewish then the dish is permitted for consumption, and if most people who would eat cooked meat in that city are not Jewish, then the pot of meat is forbidden for consumption. This means that we do not fear the meat in a pot left on the ground has been switched with non-kosher meat. Answer: In the above case the pot fell from a person’s grasp onto the ground and has been under supervision since it fell. The scholars then asked: The early sages ruled that pieces of meat found outside the area of the Holy Temple may be eaten, and this applies even if the meat was not under supervision, implying that there is no fear crows switched this meat with non-kosher meat. Answer: According to Rav, the pieces of meat are forbidden for consumption and the ruling by the early sages which permitted the meat referred to the fact that the meat is not impure, but it remains forbidden for consumption lest crows have switched it for non-kosher meat. The scholars also said that the words attributed to Rav, that pieces of meat which were not under supervision are forbidden for consumption lest crows have switched them were not explicitly stated by him, it was his students’ conclusion from a ruling he gave after an incident. Rav sat on a bridge over a river and saw a Jew washing a bovine head. While he was washing it, the head fell into the river. The Jew went to fetch a mesh basket to raise the head from the river, but what he ended up raising was two heads. Rav, who saw this, forbade the Jew from eating the bovine head lest it be non-kosher meat. His students said to him: Isn’t it possible that both heads are from kosher animals? Why did you forbid it? He answered that non-kosher meat is more prevalent that is kosher meat, and that is why he forbade it. The scholars then asked: If the above conclusion was reached after an incident which happened and the conclusion is correct, why does it have to be said to be a conclusion based on an incident involving Rav and that it is not something he explicitly said? The conclusion remains the same, after all. Answer: Because it could be said that the reason Rav forbade the bovine head is because it was in a port in which there are a majority of gentiles. The scholars then asked: How did Rav eat meat? Answer: He was stringent about supervision on the meat, from the slaughter until it was eaten, to make sure it was never out of sight. Another possibility is that he marked the pieces of meat with a special mark so he could identify it with no fear it had been switched.
Another incident involved Rav. He was near the river and saw a ship approaching. He said, “It is a holiday for the local residents, for a ship has arrived.” He went to see what was in the ship and saw there was meat hanging from hooks; he watched to make sure the meat was not switched with non-kosher meat. Even so, Rav did not eat of the meat. The scholars asked: Why did Rav refrain from eating the meat? He himself supervised to make sure it had not been switched. Answer: Since Rav’s statement that the approaching ship was a joy to the local residents was a forecast and forecasts are forbidden by Halacha, Rav refrained from eating of the meat which came on the ship he had forecast. The scholars asked: Is not a forecast like Rav’s permitted because he did not treat a sign as having influence on the action, only said that it is a good sign? A forecast is forbidden only when one uses it to decide upon an action, as Eliezer the servant of Abraham did in saying that “if the girl gives me to drink and the camels to drink I will speak to her about marriage to Isaac and if she does not offer me to drink I will not speak to her.” If so, why did Rav not eat of the meat on the ship? Answer: Because the feast was one of permission and not of obligation and Rav customarily only ate at meals of obligation.
Three rabbis relied on omens as signs for the future. Rav — would check if a ship had reached the river, this being a good sign, Samuel — would look to see what a book what was predicted of the future, and Rabbi Yochanan — would ask a child to say a verse which came to mind and from this verse would forecast the future.
(Babylonian Talmud, Tractate Chulin 95a-b)