The Gemara Zevachim (105a) discusses the status of the impurity of the carcass or meat of parim ha’nisrafim. We know that the people involved in burning them and taking them out of the camp, i.e., Jerusalem, become tamei, but what happens when someone touches them? The case with most objects which are the source of tumah, is that they make the person who touches them tamei. Here, however, that is not the case, as the Torah has only declared tumah for the one who burns it or takes it out. Nevertheless, the Gemara states that the meat imparts tumah through touch, but at a level one degree lower than a normal source of tumah.
A source of tumah is known as an av ha’tumah, the “father,” of tumah, and will make people, vessels, and food which touch it tamei. This person or object now has a derivative tumah which is known as a v’lad ha’tumah, a “child” of the tumah, . When it is a first generation derivative, as in this case, it is known as a rishon li’tumah, a first-degree derivative tumah. A rishon, in turn, can only transfer tumah to food, making the food a sheni, a second-degree derivative.
Now, when it comes to the parim ha’nisrafim, they function like an av hatumah vis-a-vis those who burn it, making them a rishon li’tumah. However, when someone is only in contact with it, she is tehorah, because it does not transfer tumah by contact. This is not unheard of in the world of tumah. For example, the non-ritually slaughtered carcass of a kosher species of bird (e.g., a chicken that died naturally) will not transfer tumah to the person who touches it, and will only transfer tumah when one swallows it. Nevertheless, says the Gemara, it – the burnt cow, and also the dead bird, will transfer tumah to food, making it a sheni li’tumah. That is, when it comes to contact, they will funciton not like an av ha’tumah, but like a rishon li’tumah, making food tamei, but leaving a person (and vessels) tahor.
What is the reason for this meat to have this status? It did not conduct tumah from somewhere else, so it should either be a source of tumah, i.e., an av, or nothing! How can it function like a rishon when it comes to touch, and why should it do so?
The answer to this is clarified in a debate between R. Meir and the Rabbis. A braitta quoted in the Gemara, states that according to R. Meir, not only do the parim ha’nisrafim function as a rishon li’tumah when it comes to touch, but even the Se’ir La’azazel, the live goat which transfers tumah to the person who brings it to the wilderness, does so. The Rabbis disagree with this. According to them, since a live animal can never conduct tumah from outside of itself, that is, can never become a rishon through touching an av, then it also cannot become a rishon in this case, and even if food touches it, the food will remain tahor.
Both positions agree, however, that the carcass of the parim ha’nisrafim will function as a rishon when it touches food. This is based on a braitta of R. Yishmael, which states that anything which will eventually, or in some other circumstance, be an av ha’tumah, will, when not functioning as an av, still function as a rishon li’tumah.
What R. Meir and the Rabbis seems to be debating, then, is how to understand this principle of R. Yishmael. They both agree that something that in another circumstance can be an av, has some power even now. The question is why? According to R. Meir, we focus on the potential tumah “inside” this object. This tumah is trying to get out, as it were, but is unable to do so. Nevertheless, some of it emerges, but in a weakened state. It only has the strength of a rishon, not of an av. According to the Rabbis, however, the potential does not come out now. It only exists as a possibility in the future. However, the object is impacted by this potential, and its state is as if it was touched by the future av ha’tumah.
[This is not a wholly unique case. The Gemara in Niddah (22a) discusses the status of a ba’al keri, a man who has experienced a seminal emission. His status is that of a rishon li’tumah. Logically, this is because the semen is an av and his body has been in contact with the semen. While this is one option the Gemara considers, the Gemara gives equal weight to the possibility that he is not a nogeya, one who touched, but a roeh, one who is tamei because of what has occured to their body, because of their status. That is, the Gemara considers it possible that this person is the source of tumah, but that he is so only with the power of a rishon. Presumably, like the case of the potential av of our Gemara, this ba’al keri‘s personal status is not weighty enough to make him an av, but because he is connected to an av – he produced the semen – his is also a source of tumah, albeit one degree lower. He is an av with the tumah of a rishon.]
The difference, then, is between a weakened tumah, a tumah that is emerging only partially, and between a tumah that, in the translation from the potential of the future to the actual of the present, exists in a derived state, as if it has touched itself as an av. This is why they debate the status of the live goat. For while a live animal can never receive tumah, it can be the source of tumah. Thus, for R. Meir, this is a source, albeit a source that is coming out weakly, that only comes out with a power of 5, not a power of 10. Such a source only transfers tumah to food; it isn’t strong enough to transfer tumah to people. In contrast, the Sages say that the goat cannot become tamei, because when it is not functioning as a source, the most it can function as is as an object that received tumah, and that does not occur to live animals.
The Gemara goes on to say that according to the Sages, a piece of a dead chicken that is the size of an olive, but smaller than the size of an egg, would not transfer tumah by touch, because food can only receive tumah if it is the size of an egg or bigger. On the other hand, it would transfer tumah by touch accoring to R. Meir, because if it is the size of an olive, that is enough to be the source of tumah, and here as well, it is functioning as a source of tumah, albeit a weakened one.
Finally, this explains a bizarre question at the end of this sugya. On 105b, Rav Hamnuna asks R. Zeira about food which became tamei through contact with these parim ha’nisrafim. Would it, he asks, be a rishon li’tumah or a sheini li’tumah? This is a very strange question indeed, for since people do not become tamei through contact with these parim ha’nisrafim, it is obvious that these parim are functioning like a rishon, and thus the food which touches them would be a sheni. This is, indeed, what R. Zeira answers him. So what was Rav Hamnuna thinking? And why did he not ask the same question according to the Rabbis?
It seems that Rav Hamnuna knew that the Rabbis would say the food is a sheni. Since the parim are functioning as if they had received tumah by contact, they are of course functioning as a rishon. However, according to R. Meir, since it is functioning as the source of tumah, that it is actual tumah that is coming out at half-power, then it is possible that the halakah would be different. Maybe this weakened tumah only has the power to transfer tumah to food and not people, but it is also possible that when it does succeed in transferring tumah it does so fully, making the food into a rishon. If it is the full tumah expressing itself partially, there is no reason to assume that it must function fully as a rishon, rather than as a parital av. Even after the conclusion that the food is a sheni, the basic assumption, that R. Meir looks at this as a weakened av, while the Rabbis look at it as a derivative tumah, remains in place.