We discussed this question in the daf this week on 78a–79a. On 78a, Reish Lakish rules on a case where someone munches on three olive-sized pieces of different prohibited items: piggul, nossar, and a piece of sacrificial meat that was tamei. If while chewing on these pieces a small piece of, say, nossar, joins with a larger piece of piggul, the nossar would become nullified (this has ramifications for whether the transgressor can become liable for lashes; see there).
On 79a, the Gemara notes that in expressing this opinion, Reish Lakish disagrees with R’ Elazar who says “prohibited items, just like mitzvah items, cannot nullify each other.” That is, in the opinion of R’ Elazar, the principle of nullification does not apply where both of the items in the mixture are prohibited or where both are mitzvah items. As an example of a “mixture of mitzvah items,” the Gemara cites Hillel’s famous Korban Pesach sandwich of matzah, marror, and Korban Pesach meat. The Gemara says that the fact that Hillel did not consider the matzah to be nullified to the bitter taste of the marror proves that, in his opinion, one mitzvah item cannot nullify another mitzvah item.
But why shouldn’t the principle of nullification apply just because both of the items are prohibited or are mitzvah objects? The principle of nullification simply says that when we have a mixture of different items that have different halachic statuses, the item that makes up the majority of the mixture defines the mixture, causing all of the items in the mixture to take on its status. So what difference should it make that all of the items in the mixture happen to have a prohibited status? Why not still use nullification to define that if, for instance, the specific type of prohibited item that makes up the majority of the mixture is piggul, then the entire mixture should be considered piggul?
To build a possible theory let’s go back to the roots of the concept of nullification. The Torah teaches us about nullification in the context of court rulings. Namely, where dissent exists in a court ruling we are to follow the majority opinion. But more than that, the Torah says that the majority opinion is legally considered the one and only opinion of the court. The minority opinion, in other words, is considered nullified in the face of the majority opinion. Now, what if all the judges ruled the same say but only a minority of them have, say, low-pitched voices, or are under 70? Why not apply nullification to those things? Of course, the answer is that those factors are obviously not salient features of the judges to be considered for nullification. The one and only factor we look at is: How did each judge rule?
So it is regarding a mixture that is made up of only prohibited or mitzvah items. Though the items do possess different particular qualities (e.g. one is piggul, one is nossar), the only truly salient feature is the general ruling that the Torah stated about each item; did the Torah say it’s prohibited or permitted? So if in terms of the prohibited/permitted status all of the items are the same, nullification is simply not relevant.
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