Parah, Chapter 7, Mishnah 10. This chapter follows the person who draws and carries the water destined to be mixed with the ashes of the parah adumah, and it examines what disqualifies that water before the ashes are added: performing a melacha while attending to it, or hesech hada'as, letting one's attention slip away from it. Our Mishnah takes up a new situation: the carrier does not keep the water himself but hands it over to somebody else to watch.
In the Mishnah's own words: "Hamoser meimav letamei", where a man delivers the water he drew into the keeping of someone in a state of tumah, the ruling is "psulin", the water may no longer be used. But "uletahor", where the one receiving it is tahor, the ruling is "ksherin", the water retains its validity. A dissenting view follows. "Rebbi Eliezer omer", Rebbi Eliezer maintains: "af letamei ksherin", handing it over even to a tamei person still leaves the water usable, "im lo asu haba'alim", provided that the owners have performed no "melacha".
Handing it to one who is tamei
A man drew this water for the purpose of sanctification, that is, to have the ashes placed into it, and he passes it to someone who is tamei to look after. Such safeguarding accomplishes nothing. A person who is himself tamei will not guard the water as it must be guarded, and so the transfer leaves the water without a proper watchman and it is disqualified.
Handing it to one who is tahor
Where the recipient is tahor, the water keeps its fitness, and here the Mishnah brings out a notable ramification. From the moment the water passes into this guardian's keeping, he takes over the owner's role with regard to it. Should the owner now go and busy himself with a melacha, no harm comes to the water at all, since the duty of watching has left his hands and rests with the other man. Conversely, a melacha done by the guardian is what will invalidate it, for responsibility has become his.
Rebbi Eliezer's view
According to Rebbi Eliezer, water entrusted even to a tamei person can remain usable, subject to one qualification: that the owner engaged in no melacha. His reasoning is that an owner who did nothing to distract himself never let his mind stray from the water, so that it stays in his own keeping notwithstanding the handover. Hence, so long as he did no melacha in the interval preceding the sanctification, the water stands; should he have occupied himself with melacha, it is disqualified.
The two halves of the Mishnah thus fit together into one idea. Everything turns on who bears responsibility for the water at a given moment. Where the water has been placed in the hands of a watchman who is tahor, the watchman's conduct is what matters and the owner's acts no longer affect it. Where the transfer was to one who is tamei and accomplishes no real safeguarding, the water still rests upon the owner's own vigilance, and Rebbi Eliezer teaches that his continued attentiveness, expressed in his refraining from any melacha, is what keeps the water fit.