Taharos, Chapter 2, Mishnah 2. This mishnah takes up a question that sounds almost like a proverb: is a person what he eats? Chazal decreed that one who eats tamei food becomes tamei himself. Beyond the prohibition of eating such food, the Sages were concerned for a very practical reason: someone who eats tamei food is liable to be careless with pure terumah, walking around with tamei crumbs still in his mouth and handling holy food as though nothing had happened. To protect terumah, they declared the eater himself tamei.
Once the decree exists, a further question arises. To what degree is such a person tamei? A decree of the Sages is normally patterned after something, and here there was a ready model in the Torah itself, the very case that opened this masechta: the neveilah of a bird. That bird does not convey tumah by contact at all, yet the moment it reaches the throat of the one swallowing it, he becomes a rishon and transmits tumah onward to whatever he touches. That is a serious status generated by an item that on its own conveys nothing.
Rabbi Eliezer: measure for measure
Rabbi Eliezer maintains that the correspondence is precise, rung for rung. He teaches "ha'ochel ochel rishon": one who eats food carrying first-degree tumah himself becomes a "rishon". So too "ochel sheini", and the man stands as a "sheini"; "ochel shlishi", and he stands as a "shlishi". Where does such a sweeping equation come from? From the swallowed neveilah of a bird. If an item that transmits nothing whatsoever through touch can still turn the one who swallows it into a full-fledged rishon, it is unthinkable that food which genuinely bears tumah would leave the person who ate it standing on a lower rung than the food itself occupies.
Rabbi Yehoshua: a technical accounting
Rabbi Yehoshua takes the opposite route, tracing the tumah through the ordinary machinery of contact. His verdict: "ha'ochel ochel rishon", and equally "ve'ochel sheini", leaves the eater at the level of a "sheini". For first-degree food no explanation is needed: whatever a rishon touches drops to sheini, and a human being is no exception. For second-degree food the path is longer. A sheini has no power to defile a person directly, but the food encounters the man's saliva, and saliva is a liquid; liquids that meet tumah are raised to rishon status. That saliva, now itself a rishon, acts upon the man and leaves him a sheini. Two different routes, one and the same result.
With third-degree food the calculation grows finer. Rabbi Yehoshua rules that one who swallowed a "shlishi" is "sheini lekodesh" but "velo sheini literumah". Since his whole method is mechanical, everything hinges on what the swallowed food was able to do to the saliva it met, and consequently on what this man is about to handle. Regarding kodesh he is reckoned a sheini, for a shlishi is potent enough to defile kodesh. Regarding terumah he is not reckoned a sheini, for a shlishi has no power over terumah at all. The degree of what he ate thus becomes relevant only in light of what his hands are now reaching for.
What food are we discussing?
The mishnah signs off with a terse closing phrase: "bechullin shena'asu al taharas terumah." The Bartenura opens it up for us. Every food item mentioned anywhere in this mishnah, the first-, second- and third-degree food that the person swallowed, is ordinary chullin that was produced under the purity requirements governing terumah, the way of those scrupulous people who chose to treat their everyday food with the sanctity and cleanness that the law demands only of terumah. The case itself supplies the proof: in plain unconsecrated food there simply is no category of third degree. Once our mishnah speaks of a man eating a shlishi, the food under discussion must be chullin prepared at the purity level of terumah.
The dispute, then, is not only about numbers. Rabbi Eliezer reads the decree as an extension of a Torah model and lets the eater absorb the exact status of what he ate, while Rabbi Yehoshua traces the tumah through the ordinary channels of contact and saliva and arrives at results that shift depending on whether kodesh or terumah is at stake.