Machshirin, Chapter 1, Mishnah 1. The name of the masechta means "preparations," and that is precisely its subject. Food does not become capable of contracting tumah on its own. Before a food can receive tumah at all, it must first undergo a hechsher, a preparation. The word hechsher here has nothing to do with kashrus in the sense we usually use it; it means readiness, being made fit. And the way food is made fit to receive tumah is by coming into contact with liquid, specifically one of the seven liquids the Torah recognizes for this purpose. Once a food has been wetted by one of those liquids, it enters the category of food that can become tamei. That single idea is the key to the entire masechta: under what circumstances does contact with liquid render food susceptible to tumah?
The first ruling of the Mishnah concerns the timing of that ratzon. It states: "Kol mashkeh shetechilaso l'ratzon," any liquid whose onset came about through the owner's willingness, "af al pi she'ein sofo" (even if its conclusion was) not "l'ratzon"; and equally, a liquid whose conclusion came about through his willingness though its onset did not, "harei zeh b'chi yutan," such a liquid is governed by the Torah's words b'chi yutan and does render food fit to receive tumah.
The Pasuk Behind the Rule
The Mishnah's expression "b'chi yutan" is lifted from a verse in Vayikra: "v'chi yutan mayim al zera," and the verse continues, "v'nafal minivlasam alav, tamei hu lachem." Taken at face value the verse teaches that should water come onto seed, and then the carcass of one of those creatures drop onto that seed, it has become tamei for you.
Here lies the decisive point. In the consonantal text the word appears as yud, tof, nun, with no vav at all, and such a spelling admits two possible vocalizations. Vocalize it "yutan" and it means the water was given, that is, it arrived through some outside cause with no action on your part. Vocalize it "yitein" and it means he gave it, that is, you were the one who applied the water. Since the letters as written sustain either reading, Chazal draw from the two readings together one composite condition: the water is allowed to come of its own accord, yet its coming must have a quality that makes it comparable to your own placing of it.
What does it mean for naturally falling water to be "as though you placed it"? It means that the wetting was pleasing to you. If water descends upon food and the result is one you wanted, that water is treated as if you had poured it on yourself. This condition of ratzon is the backbone of the whole masechta. For water that fell upon food to make that food a hechsher l'kabel tumah, the wetting must be something the owner desired.
Beginning or End Is Enough
With that in hand, the Mishnah's teaching becomes clear. Ratzon does not have to accompany the liquid from start to finish. It is enough that at one point along the way, either at the outset or at the conclusion, the arrival of the liquid was something the owner wanted. Once that is so, the liquid falls within the pasuk of "b'chi yutan," and the food it wets is a hechsher l'kabel tumah.
The Rav, the Bartenura, brings an example for each direction. First, a case where the start was with ratzon and the finish was not: someone owned a dirty pot and left it out in the open so that rainfall would collect in it and wash it out. He was thoroughly happy to have the rain drop into that pot, and that is techilaso l'ratzon. Afterward, though, the water sloshed over the rim onto his produce, an outcome he had no interest in whatsoever. The produce is nonetheless rendered susceptible to tumah, because the rain's descent into the pot was welcome to him from the very first moment.
Then the opposite situation, which the Mishnah introduces with the words "o shesofo l'ratzon," or whose end was with ratzon although its start was not: rain came down at a time when he had no wish for rain, yet once it settled on his produce he was pleased to find it there. This case too is covered by b'chi yutan, and the produce becomes a hechsher l'kabel tumah.
Liquids That Are Already Tamei
The Mishnah then adds a line that changes the picture entirely: "Mashkin temei'im metam'in b'ratzon veshelo b'ratzon." Everything said until now concerns liquid that is itself tahor, ordinary rainwater or flowing water. But if the liquid was already tamei before any of this happened, it conveys tumah whether the owner wanted the food wetted or not.
The Rav explains why with a precise formulation: tumasan v'hechsheiran ba'in k'echad, their tumah and their hechsher arrive together as one. The very same contact that renders the food capable of receiving tumah is the contact that transmits the tumah to it. The wetting and the defiling happen at one and the same instant, and therefore the question of ratzon never gets a chance to arise. If the liquid was already tamei, the food becomes tamei, regardless of whether its owner wished it to become wet.
The opening Mishnah of the masechta thus hands us the rule together with its boundary. Liquid that is tahor makes food susceptible to tumah only when ratzon accompanied it, whether at its onset or at its conclusion; liquid that is already tamei requires no ratzon whatsoever.