1. כששימש מעשה עץ שימש

The Gemara discusses a כזית of meat cut off from an אבר מן החי with the intent to use it as food (for non-Jews). As food, it contracts tumah from the אבר מן החי, provided that it first undergoes הכשר for tumah. Although it was previously metamei with טומאה חמורה – severe tumah as [part of an] אבר מן החי, and should presumably not require הכשר, Rava explains: כששימש מעשה עץ שימש – when it served as part of something that was metamei with טומאה חמורה, it served the function of wood, not food. Therefore, when it later assumes the status as food requires הכשר like any other food. The Gemara proceeds to answer five additional questions with this principle. For example, כופת שאור שיחדה לישיבה – a block of leaven which one designated for sitting loses its status as חמץ (and may be kept on Pesach), and becomes a seat susceptible to the טומאה חמורה of מושב. This tumah may be דאורייתא, yet is not an example of food transmitting טומאה חמורה, because the leaven is serving as a מעשה עץ (i.e., a seat), not as food.

1. Rebbe Shimon: food forbidden even to non-Jews cannot become tamei

In the Mishnah on Daf 127b, Rebbe Meir said that after an animal dies, its formerly dangling flesh requires הכשר before it can become tamei, while the dangling אבר is metamei as an אבר מן החי. The Mishnah concludes that Rebbe Shimon rules “it” tahor, and after extensive discussion, the Gemara concludes that Rebbe Shimon is referring to the בשר (the אבר, however, is tamei as אבר מן החי). Rebbe Yochanan explains that Rebbe Shimon infers from the passuk "מכל האוכל אשר יאכל" – from any foods which can be eaten: אוכל שאתה יכול להאכילו לאחרים קרוי אוכל – food which you may feed to others (i.e., non-Jews) is called “food,” but אוכל שאי אתה יכול להאכילו לאחרים אין קרוי אוכל – food which you may not feed to others is not called “food” regarding tumah of food. Since בשר מן החי is forbidden even to gentiles, it cannot contract טומאת אוכלין.

1. The difference between a human אבר מן החי and אבר מן המת

The next Mishnah states that a person’s dangling אבר or flesh is tahor. If he dies, the dangling flesh remains tahor, because it is viewed as if it fell off just before his death, and this Tanna holds human בשר מן החי is tahor). The formerly dangling אבר is metamei as אבר מן החי, but not as a limb from a dead body. [A Baraisa records Rebbe Eliezer’s tradition that an אבר מן המת is tahor, but Rebbe Yehoshua argues from a kal vachomer: if an אבר מן החי is tamei, an אבר מן המת is certainly tamei.] The practical difference between whether an אבר is tamei as אבר מן החי or as אבר מן המת concerns a כזית of flesh and עצם כשעורה – a bone the size of a barleycorn. A Mishnah states: כזית בשר הפורש מאבר מן החי – a kezayis of flesh which separates from a limb severed from a living [person], Rebbe Eliezer rules it tamei, while Rebbe Nechunya ben Hakanah and Rebbe Yehoshua rule it tahor. Regarding an עצם כשעורה which separated from an אבר מן החי, Rebbe Nechunya rules it tamei, while Rebbe Eliezer and Rebbe Yehoshua rule it tahor. Thus, when our Mishnah, which follows Rebbe Yehoshua, classifies the dangling אבר as an אבר מן החי, it means that flesh or bone which separates from it is tahor.