Explanation: [The Gemara] needed to explain that these are needed, lest we ask that they already argued about this. The Gemara is like this everywhere.

Explanation #1: According to this text, we discuss when they are detached, for it teaches "he sold the attached mortar." If so, this is like R. Eliezer in the Gemara;

If we discuss an attached Tanur and Kirayim (kinds of ovens), this is like R. Eliezer, who said that anything attached to the ground is like the ground;

Explanation #2 (Rashba): We discuss when they are attached, like we say (Kelim 8:7) "he scrapes off the plaster until it is on the ground";

However, its [physical] attachment is not considered attached [in Halachah]. Surely this is so, for they are Mekabel Tum'ah!

Consequence: Therefore, he did not sell the Tanur, Kirayim or millstone, even according to R. Eliezer, even though he holds that he sold the mortar.

Alternate text: According to texts that say "he sold the Tanur and Kirayim, but not the millstone", we must say that it (an oven) is considered attached. [The law of a sale] is unlike Kabalas Tum'ah;

The attachment of a millstone is not as significant as that of a Tanur or Kirayim.

Question: Why did the [Beraisa in the] Gemara say "but not the millstone", since even R. Eliezer agrees that he did not sell the millstone?

Answer (Rashba): It mentioned it because it needs to teach that when he said to him "it and everything inside", all of them are sold, even the millstone.

Explanation: We infer from the fact that it did not teach "he did not sell the key", for due to the key we conclude that it is unlike R. Meir.

This is unlike the Rashbam, who explained that now, we ask from "he did not sell the mortar."

Question: It says below (78b) that R. Eliezer and R. Meir have the same opinion, since a Mishnah (67b) teaches that R. Eliezer says "one who sold an olivepress, sold the beam"!

Answer: It does not mean that they hold exactly the same.

Proof: The Beraisa explains R. Eliezer's reason. It is called an olivepress only due to the beam.

Explanation: We can establish this even when it is open from two sides, like is normal for a pipe, e.g. they carved out a cavity in the middle to hold pebbles;

Through such a receptacle, the water [that goes through] is considered She'uvim (drawn in a Kli, and it is Pasul for a Mikveh), for it passes over the receptacle;

Source (Mikva'os 4:3 - Mishnah): One who carves out [in a pipe] in order to hold pebbles, if it is of wood, any amount (makes it considered a Kli). If it is of earthenware, [the Shi'ur is] a Revi'is;

Citation (cont.): R. Yosi says, even if it is of earthenware, the Shi'ur is any amount...

If he did not carve out, even if it is closed on three sides, it is not considered a Kli, like the Rashbam proved from the Mishnah (Mikva'os 4:2) "one who placed a board under a pipe, if it has a brim in four directions..."

It is unreasonable to establish this to discuss a pipe closed on every side.

Limitation: This is only when the water falls from the pipe [directly] to the Mikveh;

However, if it falls from the pipe to the ground, and from the ground to the Mikveh, this is Hamshachah, and it does not disqualify a Mikveh if there are already 21 Sa'im of rainwater inside, like the Mishnah in Temurah (12a) says.

Explanation (Rashbam): However, Rabanan who argue with him are not the Rabanan mentioned above "this is not R. Eliezer, and not Rabanan." (Rather, our Beraisa is unlike a different teaching of Rabanan.)

Remark (Ri): He was forced to say so, for above he explained "he sold a carved out mortar", i.e. he attached it and afterwards carved it out, "but not an attached [mortar]", i.e. he carved it out and afterwards attached it;

Inference: Rabanan distinguish between what was carved out and afterwards attached, and what was attached and afterwards carved out. If so, [our Beraisa] can be like [those] Rabanan!

Rebuttal: There was no need to explain like this. We can explain that "carved out" means that it was never detached. He carved it out in a rock coming out of a mountain;

"Attached" is whether it was attached and afterwards carved out, or carved out and afterwards attached.

Explanation: Now we are thinking that we discuss an attached hive. We must explain that "it is like land" to be acquired through money, a document or Chazakah;

However, in the end, that we conclude that the reason is because it is equated to a forest, this (it is like land) applies to the honey in the hive. We are not concerned that the hive is attached.

Observation (Rabbeinu Yitzchak): Now, we could have asked "also according to you [it is difficult], who do not establish the reason to be due to the Hekesh to a forest, rather, because the hive is attached through mud, why is one liable for extracting the honey on Shabbos?"

This is like taking bread from the oven (they used to bake it stuck to a wall of the oven). One is not liable for this, even though the oven is attached!

Also regarding Tum'ah, the honey is Mekabel Tum'ah even in its place, since it does not require intent!

Also regarding Pruzbul, it should suffice [to explain why one may write a Pruzbul based on it, due to the place it lies on, like it says in Gitin (37a) about a flowerpot with a hole!

Rather, you are forced to say that it rests on pegs, like it answers there. If so, R. Eliezer's reason is not because it is attached through mud!