Challah, Chapter 4, Mishnah 2. In the previous mishnah we discussed a case where a woman has two different batches of dough, each of them less than the measure that requires challah, and the question is whether they combine with one another. We learned that if they are of the same kind - from the same type of flour - they do combine and become obligated in challah, and if they are not of the same kind, they do not combine. We also mentioned that if she kneaded them completely together, they combine even if they are of different types.
Defining "a kind with its own kind":
Our mishnah comes to define what "a kind with its own kind" means and which kinds are considered a single kind. As we mentioned at the beginning of the tractate, five species of grain are obligated in challah: wheat, barley, oats, rye and spelt. The mishnah asks: "Eizehu min b'mino?" - which is a kind with its own kind? And it answers with two rules:
"HaChittim einan mitztarfos im hakol ela im hakusmin" - wheat combines only with spelt.
"HaSe'orim mitztarfos im hakol chutz min hachittim" - barley combines with all the other kinds, except for wheat.
These two rules raise an interesting question: if wheat combines only with spelt - implying that spelt and wheat are a single kind - how can it be that barley combines with everything, including spelt? The commentators explain that spelt is an intermediate kind: it has aspects in which it is a type of wheat, and it also has a dimension of being a type of barley.
The view of Rabbi Yochanan ben Nuri:
Rabbi Yochanan ben Nuri says: "She'ar haminim mitztarfin zeh im zeh" - the remaining kinds, apart from wheat and barley, that is, oats, rye and spelt, combine with one another among themselves.
This requires clarification: what is the understanding of the first Tanna, who disagrees with Rabbi Yochanan ben Nuri? It appears from his words that these kinds do not combine with one another in their own right. There is a certain difficulty in this, and therefore some understand that the first Tanna remained in doubt about the matter, and Rabbi Yochanan ben Nuri came and ruled with certainty that they do indeed combine with one another.
In summary: in this mishnah the Sages defined what "a kind with its own kind" means regarding the combining of doughs: wheat combines only with spelt, and barley combines with all the kinds except for wheat, because spelt is an intermediate kind that leans in both directions. Rabbi Yochanan ben Nuri adds that the remaining kinds combine with one another, and regarding the opinion of the first Tanna the commentators disagree as to whether he denied their combining or remained in doubt.