Taharos, Chapter 2, Mishnah 7. This chapter charts the degrees of tumah in food, asking how far tumah travels once it has entered, and whether the answer differs between chullin, terumah and kodesh. Here Rabbi Elazar offers his own systematic chart of the whole subject.
One principle should be held in mind throughout, and it is drawn from maseches Parah, chapter eight, mishnah zayin: anything with the strength to disqualify terumah renders liquid tamei as a techilah, a rishon. Solids step downward with each contact, but liquids jump upward. When our mishnah speaks of something being "metamei mashkeh," it means that the liquid it touches becomes a full-strength rishon, able in turn to render something else a sheni.
"Rabbi Elazar omer: Sheloshtan shavin." Rabbi Elazar states that all three are equal. Which three? "Ha'echad shebakodesh, veshebaterumah, veshebachullin." A rishon in the realm of kodesh, a rishon in the realm of terumah, and a rishon in the realm of chullin. The three foods are certainly not identical: they differ from one another in their sensitivity to tumah and taharah. Rabbi Elazar's point is that this difference does not affect their strength as transmitters. Whatever the food, a rishon is a rishon, and its power to impart tumah is exactly the same.
What does that uniform strength actually amount to? Everything depends on what the rishon comes into contact with. Kodesh is the most delicate of the three, and so the mishnah teaches "metamei shenayim ufosel echad bakodesh": a rishon, whatever its origin, produces two further links that truly carry tumah onward, a sheni and after it a shelishi, and beyond them a fourth item that is disqualified only, tamei in itself yet with nothing left to hand on. In the domain of terumah ("baterumah") the sequence is trimmed, for terumah is not as delicate as kodesh: "metamei echad ufosel echad", one link that is tamei and still transmits, and then a second that is nothing more than invalid. As for chullin, "ufosel es hachullin": the rishon does no more than invalidate it. The chullin has indeed contracted tumah, but it is granted no standing to carry that tumah any further.
The mishnah now turns to the second degree. "Hasheni shebekulan": a sheni, in whichever of the three categories it is found, again acts with one uniform strength, and it makes no difference whether the tamei item itself is chullin, terumah or kodesh. Meeting kodesh, "metamei echad ufosel echad bakodesh", it yields a single link that is tamei and still capable of passing tumah along, while the link after that is disqualified and no more. Meeting chullin, the mishnah speaks of liquid: "metamei mashkeh chullin", chullin liquid that it touches contracts tumah, exactly as the principle about liquids dictates. And with terumah, "ufosel ochlei terumah", solid terumah that it touches descends to the third rung, where the sequence comes to an end, since a shelishi has nothing left to pass downward.
Finally the third degree. "Hashlishi shebekulan": whichever of the three categories an item of the third degree belongs to, the only realm still within its reach is kodesh. Should it come into contact with kodesh liquid, "metamei mashkeh kodesh", that liquid contracts tumah and, following the way of liquids, acquires the strength of a rishon that passes tumah onward. Should it touch solid kodesh, "ufosel ochlei kodesh", the food is invalidated: it is tamei, yet the chain terminates there, and nothing further receives tumah from it.
This is a mishnah of pure tumah and taharah algebra, and it repays the effort of drawing it out. Set the three categories across the top of a page, list rishon, sheni and shelishi down the side, and fill in the boxes; Rabbi Elazar's system becomes strikingly clear once it is seen laid out in a chart.