It is due to the fact that it is the typical manner of creeping animals to expose the contents of a vessel so that they may drink. Therefore, the exposure of the water is attributed to a creeping animal or to a ritually pure person. By contrast, in a case where he left the vessel exposed and found it covered, the concern is that it was an impure man who covered it, since it is not the typical manner of creeping animals to cover exposed vessels. Evidently, with regard to prohibition or ritual impurity, there are circumstances of uncertainty when the ruling is lenient.

Alternatively, it can be inferred from the baraita that the reason the contents of the vessel are impure or disqualified, respectively, is that he left it exposed and came back and found it covered or that he left it covered and came back and found it exposed. But if he found the vessel just as he left it, there is neither impurity nor disqualification.

But in a situation of uncertainty where he left exposed water and then came and found the vessel exposed, the water is forbidden under all circumstances. Learn from it that danger is more severe than prohibition. The Gemara affirms: Indeed, learn from it.

We learned in a mishna there (Terumot 8:4): Three liquids are forbidden due to exposure: Water, wine, and milk. How long shall they remain exposed and their contents will be forbidden? It is a period equivalent to the time necessary so that a snake could emerge from a proximate place and drink. And how far away is considered a proximate place? Rav Yitzḥak, son of Rav Yehuda, said: Even a period equivalent to the time necessary so that a snake could emerge from beneath the handle of the vessel and drink.

The Gemara asks: If it is only the time necessary for the snake to emerge and drink, doesn’t one see the snake drink, in which case there is no uncertainty? Rather, it is a period equivalent to the time necessary for a snake to emerge from a proximate place, drink, and return to its hole. If one left exposed liquid unattended for that interval, it is possible that the snake drank the liquid unseen by the owner of the liquid.

§ It was stated: With regard to one who slaughters an animal with a knife that was afterward found to be notched, Rav Huna says: Even if, after the slaughter and before the knife was examined, he broke bones with the knife all day, the slaughter is not valid, as we are concerned that perhaps the knife became notched on the hide of the neck. And Rav Ḥisda says: The slaughter is valid, as perhaps it was on the bone that he broke with the knife after the slaughter that it became notched.

The Gemara asks: Granted, Rav Huna stated his opinion in accordance with his halakha cited earlier (9a): An animal during its lifetime exists with the presumptive status of prohibition until it becomes known in what manner it was slaughtered. But as for Rav Ḥisda, what is the reason for his ruling that the slaughter is valid? The Gemara answers that Rav Ḥisda could have said to you: A bone certainly notches the knife, but with regard to hide, it is uncertain whether it notches the knife and uncertain whether it does not notch it. This is a case of certainty and uncertainty, and the principle is that an uncertainty does not override a certainty.

Rava raises an objection to the opinion of Rav Ḥisda to support the opinion of Rav Huna, from a baraita: If one immersed and emerged from the ritual bath and an interposing item was later found on him, then even if he had been engaged in handling that same type of item for the entire day after his immersion, the immersion does not fulfill his obligation. This is so until he will say: It is clear to me that this interposition was not on me beforehand. And here it is a case where he certainly immersed, and it is uncertain whether the interposition was on him at that time and uncertain whether it was not on him, and nevertheless, contrary to the opinion of Rav Ḥisda, the uncertainty overrides the certainty.

The Gemara rejects that proof: It is different there, as it can be said: Establish the status of the impure person on the basis of his presumptive status of impurity, and say that he did not immerse properly.

The Gemara challenges: Here too, establish the status of the animal on the basis of its presumptive status of prohibition and say that it was not slaughtered properly. Why does Rav Ḥisda rule that it is permitted? The Gemara explains: That status has been undermined, as the slaughtered animal is before you. There is no indication that the slaughter was not valid, and most slaughtered animals are slaughtered properly.

The Gemara challenges: Here too, in the case of immersion, the status of impurity is undermined, as the person who has immersed is before you. The Gemara explains: The case of immersion is different, as a flaw developed in the presumptive validity of the immersion, since there is an interposition.

The Gemara challenges: Here too, a flaw developed in the presumptive validity of slaughter, as the knife is notched. The Gemara explains: In the case of slaughter, the knife became flawed, but the animal did not become flawed. Therefore, the animal assumes the presumptive status of permissibility. By contrast, in the case of immersion, the interposition was found on the person, thereby nullifying his presumptive status of purity.

The Gemara raises an objection to the opinion of Rav Ḥisda from a baraita: The slaughter of a bird is valid with the cutting of one siman, the windpipe or the gullet. Therefore, if one cut the gullet, and the windpipe was displaced thereafter, the slaughter is valid. If the windpipe was displaced and one cut the gullet thereafter, the slaughter is not valid.

If one cut the gullet, and the windpipe was found displaced, and he does not know whether the windpipe was displaced before the slaughter or whether it was displaced after the slaughter; that was the incident that came before the Sages, and they said: In any case of uncertainty with regard to slaughter, the slaughter is not valid.

The Gemara asks: With regard to the expansive formulation: With regard to any case of uncertainty with regard to slaughter, what does it serve to add? Does it not serve to add a case like this one where there is uncertainty whether the knife was notched before or after the slaughter? The Gemara answers: No, it serves to add a case of uncertainty whether he interrupted the slaughter in the middle, or uncertainty whether he pressed the knife on the simanim. If he did either, it invalidates the slaughter.