Parah, Chapter 9, Mishnah 4. This chapter takes up the many ways in which mei chatas, the purification waters of the parah adumah, can lose their status and become unfit for use. Our Mishnah deals with a case that sounds almost innocent: a person looks at the vessel of mei chatas and thinks about drinking from it. Does a thought like that already ruin the water, or does something more have to happen?
The case is introduced with the words "hachosheiv al mei chatas", one who forms a plan regarding the purification water, "lishtos", namely, to drink it. Rabbi Eliezer's verdict on such a person is a single word: "pasul", the water is spoiled. Rabbi Yehoshua takes issue and rules "kesheyateh": it goes out of use only from the moment the man tips the vessel.
What does tilting mean here? The person inclines the container toward himself so that he will be able to drink from it comfortably. That physical act, according to Rabbi Yehoshua, is what disqualifies the water, and it disqualifies it even if in the end he never swallowed a drop. The thought by itself, however, accomplishes nothing; it is the tilting that ruins the water.
Rabbi Yosei's distinction
Rabbi Yosei now narrows the whole argument. He asks "bameh devarim amurim", to which situation does this machlokes apply, and answers "bemayim she'einam mekudashim": only where the ashes have not yet been placed into the water. "Aval bemayim hamekudashim", where the ashes have already been added and the water carries its full status, each Tanna pushes his position one stage further along. Rabbi Eliezer now teaches "kesheyateh", nothing is lost until the vessel is tipped, while Rabbi Yehoshua teaches "kesheyishteh", nothing is lost until the man has swallowed. Whatever each of them demanded for the plain water, he demands the next, stronger act once the ashes are in.
The Mishnah closes: "Girgeir, kasheir." If he poured the water into his mouth without bringing his lips to the vessel, the remaining water is fit.
The underlying principle
There are opinions that explain this entire disagreement on the basis of hesech hada'as, the removal of one's mind from guarding the water. Mei chatas must be watched over, and once a person's attention has left it, it is no longer usable.
Along these lines, Rabbi Eliezer disqualifies unsanctified water through intention alone, because the moment a person plans to drink it he has evidently turned his mind away from safeguarding it. With sanctified water, though, no act means no disqualification: even though he thought about drinking, we assume that he thought better of it and continued to guard the water, since with sanctified water a person is afraid of losing something so precious.
Rabbi Yehoshua, for his part, maintains that even unsanctified water is disqualified only through an act, such as tilting the vessel in order to drink from it. Short of such an act, we do not say that he removed his mind from guarding it at all.
The lesson of this Mishnah is how much care mei chatas demands. A vessel of purification water is not simply a container of liquid standing in a corner; it is under a person's watch, and the slightest turn of the hand toward using it for himself can undo everything that was invested in preparing it.