Taharos, Chapter 8, Mishnah 7. As the chapter draws toward its close, the Mishnah leaves behind the specific cases it had been treating and turns to broader, more klali principles of tumah and taharah. Our mishnah is one of those general discussions, and it revolves around a case whose tumah is not biblical at all, but a decree of the Chachamim.
The background: Keilim, chapter 25
To follow this mishnah, we need a piece of information that comes from a different tractate entirely. Not our Mishnah in Taharos, but the Mishnah in Keilim, in the twenty-fifth chapter, mishnah six, which discusses what happens when the outer surface of a vessel becomes tamei through contact with liquid. Mi'De'oraisa such a vessel is not tamei; liquid does not convey tumah to a kli by Torah law. The Rabbanan, however, made a gezeirah on the outside of the vessel: if the outside is exposed to tamei liquid, the outside alone is reckoned tamei, while the inside remains tahor, and so do the handles and the rim. If, by contrast, the inside of the kli becomes tamei, the entire vessel is tamei, inside and out.
The key point is that the tumah of the outer surface is only de'Rabbanan. And once we are dealing with a tumah that the Rabbanan themselves instituted, it is entirely possible that the ordinary rules will not apply in the ordinary way. That is precisely the question our mishnah addresses.
The three opinions
"Achorei keilim shenitme'u bemashkin": the outer parts of vessels that became tamei through liquids. What power do they have to pass tumah along?
Rabbi Eliezer says: "metamei es hamashkin ve'ein poslin es ha'ochel." Such a vessel renders liquids tamei, but it does not disqualify food; terumah that touches it is not rendered pasul.
Rabbi Yehoshua says: "metamei es hamashkin ufoslin es ha'ochel." He is somewhat more machmir: the vessel renders the liquids tamei and it also disqualifies the food.
Then comes Shimon achi Azaryah, who says: "lo chach velo chach," it is neither the one way nor the other. Both of the previous opinions, in his view, have framed the question wrongly. "Ela mashkin shenitme'u ba'achorei keilim, metamei echad ufosel echad." Rather, the liquids that became tamei on the outer surface of the vessel make one thing tamei and disqualify one thing further along. With this we return to the familiar chain of degrees: the liquid that touched the outside of the kli becomes a rishon; it renders what it touches tamei; that item in turn renders the next item pasul; and since that last item is a shlishi, it can carry the chain no further.
The liquid's retort
The mishnah then puts a remarkable statement into the mouth of the food: "harei zeh omer: metam'echa lo tim'ani ve'atah timateni." The solid food says to the liquid: the very thing that made you tamei, the outer surface of the vessel, has no power to make me tamei, and yet you, who received your tumah from it, do make me tamei.
This is one of the genuine puzzles in the way this halachah operates. The outside of the vessel cannot render food tamei, yet it can render liquid tamei; and that liquid, once tamei, turns around and renders food tamei. The recipient ends up with a reach that the source itself never had. Such is the shape the Chachamim gave to their gezeirah, and the mishnah lets the paradox stand openly for us to notice.