Yadayim, Chapter 1, Mishnah 5. This is the closing mishnah of the first chapter, the chapter that sets out the requirements of netilas yadayim: the water, the vessel, and the manner of pouring. Here the mishnah turns to the one doing the pouring. Must the person who pours the water onto your hands be a qualified adult? Must it even be a person at all?
The mishnah gives a sweeping ruling: "Hakol kesherim litein layadayim," anyone at all may be the one who pours water over another person's hands. The mishnah then spells out how far that reaches, "afilu cheresh, shoteh v'katan": the one who neither hears nor speaks, the one whose mind is not sound, a child under bar mitzvah. None of them is excluded. The halachic capacity of the pourer plays no part in the requirement whatsoever.
The mishnah widens the ruling still further. "Maniach chavis bein birkav v'notel": one may grip a barrel with his legs, pour from it in that position, and his hands are validly washed. "Mateh chavis al tzidah v'notel": he may lean the barrel over so that it comes to rest on its side, and wash with the water that spills from it. "V'hakof notel layadayim": even a monkey may do the pouring over a person's hands.
A dissenting voice closes the mishnah. Rabbi Yose is "posel bishnei eilu": he invalidates two of the arrangements just listed, the barrel left leaning on its side and the monkey. The Bartenura lays out precisely what stands behind each objection.
Koach Gavra
The principle behind the entire discussion is that the water of netilas yadayim must arrive on the hands through the application of force, koach gavra. That is why one cannot fulfill netilas yadayim by holding out his hands in the rain or sticking them under a waterfall. The water has to be delivered, not simply encountered.
Turning first to the barrel held by the legs, the Bartenura notes that a man's knees "mesay'in lishpoch," they take an active part in tipping the water out, and that qualifies as koach gavra in the fullest sense. This, the Bartenura points out, is why Rabbi Yose voices no objection here: on this case there is no dispute at all, and the washing is good.
On the barrel leaning on its side, the Bartenura stretches the case to its outer limit. "Va'afilu halach v'yashav lo achar kach": the man may have walked away and sat himself down, while the barrel continues "shofeches mayim," discharging water, "kol hayom kulo," straight through the day. To the Tanna Kamma the flow still traces back to him. It comes about "machmas hatayaso," because of the tilt he gave it, and so it is reckoned as koach gavra even hours later.
The Monkey
About the monkey the Bartenura observes that it "yeish lo yadayim k'dei adam," its limbs function as human hands do, so it is physically able to lift a vessel and pour. And on the first opinion that is all that is called for: "d'koach nosen ba'inan, koach adam lo ba'inan." What must be present is force from some agent delivering the water; nowhere did the halachah insist that this agent be a human being.
Rabbi Yose is unwilling to accept either case. Concerning the monkey, the Bartenura explains that his objection is "d'ba'inan koach adam," the force applied has to come from a human being, "v'leka," and with an animal that element is absent. And where the barrel lies tilted on its side, water that keeps running out by itself is, in his view, "lav koach gavra," not an exertion of human strength, notwithstanding the fact that a person set the whole thing going.
So the chapter closes on a fine question that has stayed with the halachah ever since: how far does a person's act reach? The Tanna Kamma sees the man's hand in the water that pours all day from the barrel he tilted, and is content with any agent whose force delivers the water. Rabbi Yose wants a human being pouring, here and now.