Passing On Tuma

Let's back-up and look at the first chakira in מנחה טהורה. Tuma can be passed from object to object, starting with an Av Hatuma and progressing through various levels of Vlad Hatuma. For instance:

1.  a Sheretz Met is an av hatuma
2. A kli that touches it can become rishon
3. Food that is put in the kli becomes sheni
4. Truma that touches the food becomes shlishi
5. Kodshim that touch the Truma becomes revii

Rav Wolf points out that there are two ways two understand this passage of Tuma from object to object:

1. Tuma Weakens as it is Passed On

Tuma weakens as it is passed on. The various categories of Av and Vlad hatuma represent the current strength of the Tuma.

2. Avot and Toldot

 

Only avot pass on tuma. Toldot are tamei but cannot pass it on. Truma/Kodshim become pasul but not tamei. Rav Aharon Lichtenstein suggests this explanation based on the Gemara in Bava Kama(ב.) that compares levels of tuma with Avot and Toldot from Melachot Shabbat(שיעורי הרא"ל טהרות עמ' 85). 

Difficulties with Both

Rav Wolf ultimately points out a number of difficulties with both explanations. For instance, if Tuma weakens as it is passed on, then why the distinction between Avot and Toldot? On the other hand, if only avot pass on Tuma, then how can we understand חרב כחלל where Av status is passed on to the kli?

In any case, I wanted to bring-up this chakira now since it is so basic and will no doubt be relevant in numerous places in the future.