Rambam - 1 Chapter a Day

Others interpret the Rambam’s source (Ohalot 6:7) as referring to a pillar standing in free space.

I.e., an ornate architectural design on the top of the pillar for decoration that projects beyond its boundaries. This was common in the Greek and Roman period and indeed, in prior eras as well. See I Kings 7:26 [the Rambam's Commentary to the Mishnah (Ohalot 6:7)].

The flower is not considered as an ohel, because it does not project a handbreadth beyond the pillar.

See Chapter 7, Halachah 4.

The Ra’avad maintains that if there is a cubit of free space, the impurity is enclosed within and does not spread outward, as stated in the notes to Chapter 12, Halachah 6. As mentioned there, the Rambam does not accept this position.

If there is not a space that large, the wall is governed by the laws mentioned in Chapter 24, Halachah 5. The storey adjacent to the impurity is impure, but the upper stories are pure.

I.e., the same wall runs several storeys high.

In his Commentary to the Mishnah (Ohalot 7:1), the Rambam states “even if the building was constructed to the maximum height possible.” Diagram

And the impurity remains contained in the second storey without spreading to the third.

But is a part of a different structure.
The Ra’avad objects to the Rambam’s ruling based on his understanding of the fmal clause of the above mishnah. As the Kessef MisJmeh explains, it is evident from the Rambam’s commentary there that he interprets that mishnah differently than the Ra’avad.

The walls of the structures in the Talmudic period were quite thick (often a cubit or more) and it was common for the inhabitants to hollow out a portion to use for storage and the like.

In his Commentary to the Mishnah (Ohalot 6:7), the Rambam explains that this term refers to storage compartments hewed out from the walls of a house. They have doors and thus resemble a chest of drawers. The Ra’avad offers a different definition of this term.

The Ra’avad states that the Rambam’s ruling contradicts his source (Tosefta, Ohalot, the conclusion of ch. 7). The Kessef Mishneh opines that there could be a scribal error in the text of the Mishneh Torah and the proper version is “the house is impure,” for it is the nature of impurity to seek to depart even if the doors are closed. Alternatively, he explains that we are speaking about a hole that opens to the outside.

I.e., there is not a cubic cubit of free space around it.

As mentioned in Chapter 24, Halachah 4, with regard to impurity found in the walls of a structure.

Its door intervenes and prevents the ritual impurity from entering.

In his Commentary to the Mishnah (op. cit.), the Rambam spells out exactly the point of this ruling. Say a wall was four cubits thick and two cubits were hewed out as a pardesik. If impurity were found in the wall a handbreadth behind the pardesik, it is only a handbreadth from the inner space of the house when counting the pardesik as part of the house. Hence the house would be impure. If, however, the pardesik is considered as a solid mass, the impurity is considered in the outer portion of the wall and the house is considered as pure.

A place that is not under the lintel of the doorstep.

This applies even if the door to the house is open.

In the same way as the laws applying to impurity in the walls of a house (ibid.).

Since the impurity is under the lintel, it is considered as part of the inner space of the house.

For as long as the dog is alive, the flesh from the corpse that it ingested does not impart impurity to a house (Chapter 20, Halachah 2).

After three days, the flesh is considered to have been digested and no longer imparts impurity (Chapter 20, Halachah 4).

I.e., we consider the place where the dog’s stomach is located [the Rambam’s Commentary to the Mishnah (Ohalot 11:7).

The commentaries recall Chapter 18, Halachah 2, which states that when a person was lying on a doorstep, part of his body being inside the house and part being outside the house and impurity was positioned in the portion outside the house, the house is impure, because a person is hollow and his upper portion is a handbreadth high. Seemingly, this same concept should apply with regard to a dog. It is possible to say that, in contrast to a human, it is unlikely that a dog will have open space of a cubit within his body.

The corpse of the fetus can impart impurity, but since it is within her body and she is alive, it does not. As soon as her womb opens, as the Rambam proceeds to explain, the fetus imparts impurity.

More particularly, the needle used for the strands of the warp [the Rambam’s Commentary to the Mishnah (Ohalot 7:5)].

To discharge the fetus.

Once the fetus emerges, it imparts impurity even if the head is smaller, as indicated by the following halachah.

Thus even if the fetus is not visible, we assume that the fetus was crushed and dissolved [the Rambam’s Commentary to the Mishnah (Nidah 3:4)].

The fact that he did touch him in the mother’s womb is insignificant, as stated in the following halachah.

In his Commentary to the Mishnah (Ohalat 7:6), the Rambam writes:
A viable fetus [seeks to] emerge itself and assists its discharge, if its formation has been completed. It does not allow itself to be discharged until [its formation] is completed. A stillborn fetus, by contrast, has no nature... holding it back. Instead, the body will discharge it like it discharges a dangerous substance.... When the first fetus emerges stillborn, we say that [the body’s] nature discharged it, but did not discharge the living fetus. On the contrary, it withheld it. After the stillborn was discharged, [we say that] the living fetus was aroused to emerge. [Thus there was an interval between their emergences and] it did not touch the stillborn when it emerged.
If, however, the living fetus emerged first, the stillborn one will certainly be drawn after it, for there is nothing to hold it back. It will thus emerge when the womb opens.
The Ra’avad differs with the Rambam’s interpretation, explaining that the mother will contract impurity from the dead fetus and then impart impurity to the living fetus. Alternatively, the mother will impart the impurity associated with birth and impart that impurity to the living fetus. These issues are discussed by the Tasafat Yam Tav and other commentaries to the above mishnah.

Chapter 2, Halachah 1.

I.e., the impurity that comes from direct contact with a corpse. She also contracts the impurity that results from birth (see Hilchot Issurei Bi’ah 5:13-15).

At which point, it is considered to have entered the world and act as a source for impurity.

See Chapter 1, Halachah 3.