Siman 7

It has already been explained in siman 3 that we require human strength in shechitah. Even if the person is not holding the knife against the animal's neck and slaughtering, but even if the knife slaughters on its own and only comes through human strength, as was written there. Therefore, our sages said [16a]: One who slaughters with a <i>mokhni</i>, his shechitah is kosher. That is to say, a <i>mokhni</i> is a wheel of wood or of stone, provided that the wheel is not attached to the ground but rather detached from the ground, like the wheel of potters who have a <i>mokhni</i> that they rotate when they engrave the earthenware vessel [Rashi]. And he laid the knife in it with its edge upward and rotated it, and the knife went across the animal's neck and slaughtered it, even though at the time of the shechitah the person did not touch the wheel. For since the person rotated it with his hand or his foot, everything the wheel does during this rotation is called "from human strength." And the matter is simple that the animal's neck should not be on top, so that it does not come to <i>derasah</i>, as was written in the previous siman regarding one who fixed a knife into a wall; see there [Bach].

And know that from the language of our teacher, the Beit Yosef, who wrote "a person may fix a knife to a wheel etc.," it is explained that even ab initio it is permitted to do so, and this is the opinion of the Rashba and the Tur, see there. However, the Rosh, of blessed memory, wrote that only post facto is it kosher, but ab initio it is forbidden, for we decree on account of a water wheel that does not come through human power, as will be explained. Also the language of the Rambam, of blessed memory, in chapter 2, law 13, implies this, and it is also proven from the language of the Gemara that we brought [and the Rashba and the Tur hold that according to the conclusion which stands on a potter's water wheel, it is permitted ab initio]. And all the great Acharonim agreed to this opinion, and thus we hold, that ab initio one should not do so.

If the water turns the wheel and the neck of the animal is placed against the wheel and it is slaughtered by the wheel, its slaughter is invalid, for there is no human power here. And if the wheel was stationary, for example, if there was a board preventing it from turning, and a person removed the board and the wheel began to turn and the animal was slaughtered, its slaughter is valid, for this turning comes from human power. In what case does this apply? During the first rotation, meaning that the wheel turned in its entirety one time, for this comes from human power. But during the second rotation, and certainly from then on, the human power has already departed, and the person was only a cause for it to turn, but his actual power was exhausted in the first rotation, for from here on the water turns it. And if it was slaughtered during the second rotation, and certainly from then on, its slaughter is invalid. And it is not similar to a machine without water, where everything it turns is considered human power since there is no other power in it, but with water, it turns by the power of the water. [And the Pri Megadim wrote this is due to the reason of "the power of his power," see there; and this requires further study, for regarding damages and exile we hold that it is like his own power, and see the Tevuot Shor siman 3, subsection 4. And this is not necessary, for the reason is as I wrote, that it is nothing more than a grama, as is explained from Rashi there, starting with the words "But with the second power etc.," that it is a mere grama. Thus far his words. And that which he changed his explanation from slaughter to that of Rabbi Papa, it is all according to the matter, but the reason is one; see there and examine it closely.]