A particular Mishnah (8:8) learnt this week is understood quite differently by the Rishonim.
The Bartenura understands that the Mishnah discusses a small pieces of dough resting on a kneading trough. The first case is where the trough is inclined. If there are three pieces in a row one above the other and the bottom piece is wet, then the trough remains tahor. Recall, that for food to pass on tumah, it must be the size of a kebeitzah (an egg). The Bartenura explains that even if the pieces are in contact with one another, they do not combine to make that minimum volume. The pieces of dough would need to "bite" into one another, such that if they were pulled apart, bits of one would be pulled away with the other. Had they combined, then they would have made the liquid tameh which, by way of a rabbinic decree, would have made the trough tameh. That is indeed that case if there were two pieces of two that made the volume of a kebeitzah. R' Dosa maintains that even in that case, they would not combine unless the two pieces were pressed together.
The Mishnah continues however that if the trough was resting flat, then any number of small pieces could combine. The reasons is that the liquid on which all the pieces rest is stationary and is able to combine the pieces. R' Dosa disagrees in that case as well.
The Mishnah Achrona explains that we learn a number of points from this Mishnah. From the final case we learn that only liquid that is stationary can combine different pieces of dough. From the first cases however, we learn that even if the pieces are on an incline, the surface liquid can combine two pieces. How so? Citing the Raavad he explains if there is a kebeitzah volume of pieces in direct contact with liquid, then they can make the liquid tameh. This is the case where there were two pieces totalling a kebeitzah. There was a kebeitzah volume of tameh food in contact with the surface liquid, which therefore made it tameh, thereby making the trough tameh. The reason why in the first case, with three pieces, the trough remained tahor, was because only one piece was wet and only two pieces where in contact with the liquid, which was still less than a kebeitzah.
The Rambam however explains that in the first case it is the trough that was wet (and not one of the pieces of dough). Since it is on an incline it can only combine two pieces, but not three. The Mishnah Achrona understands that this is even if the pieces were not in contact with one another. Kehati explains that the reason why they can combine when there are two pieces is because the liquid between them is in contact with tameh food that (together) is the size of a kebeitah.
A ramification of this debate may be found in another Mishnah (9:8). If a dead sheretz is found in a mill used for crushing olives prior to pressing, then only the olives in contact with the sheretz are tameh. The Mishnah adds that if there is already oil collecting beneath the olives then they are all tameh. The reason is that the sheretz would make the olives that they are contact with tameh; they would be a rishon le'tumah. Those olives would make then make the liquid tameh. As mentioned above, by way of the rabbinic decree, tameh liquids always became a rishon le'tumah. Consequently they would make the remaining olives also tameh. Exactly what level of tumah might depend on how we understand our Mishnah.
The Mishnah Achrona explains that according to the Bartenura, the pieces of dough ordinarily cannot combine unless they bite into each other. In our Mishnah, we learn that the liquid serves that purpose. Consequently, that liquid effectively turns the pieces into one mass. That being the case, the liquid would make all the olives considered one mass. This would mean that from the outset they would all be considered as having come into direct contact with the sheretz and be a rishon le'tumah. That reasoning was not necessary for the Rambam. The liquid definitely combined them for tumah transfer, but it does not necessarily make them considered one mass. Consequently, in that Mishnah, the other olives may have only became tameh as a result of the tameh liquid, and would therefore be a sheni le'tumah.
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