Commentary for Avodah Zarah 94:20

The wall that is shared by the Jewish homeowner and the idolatrous shrine is considered to be jointly owned. The half that is next to the shrine is forbidden for Jews to use and the half that is next to the Jewish house is permitted.

According to Deut. 7:26 a Jew must abhor idolatrous objects. The word for “abhor” is “sheketz”, which is the same word used for an impure creeping thing in Leviticus 11:31. From here the mishnah learns that just as creeping things transmit impurity, so too do idolatrous objects. The type of impurity that a creeping thing imparts is contact impurity. It does to impart impurity to one who carries it (without touching it). Contact impurity is a lesser type of impurity than carrying impurity.
Rabbi Akiva learns the impurity of idolatrous objects from Isaiah 30:22, which explicitly compares idolatrous objects to menstruating women, both being impure and imparting impurity to others. A menstruating woman imparts impurity both through contact and through carrying. So too, according to Rabbi Akiva, do idolatrous objects. In other words Rabbi Akiva holds that the impurity of idolatrous objects is more serious than that of the creeping thing.

The mishnah states that if his wall falls down, he should rebuild it four cubits within his own property. This way he is not building a wall for the shrine. But the problem is that this extra four cubits will now, de facto, become the property of the shrine.
The solution, the Talmud suggests (somewhat grotesquely) is that he use it to make a toilet. The idolatrous Temple will not get much benefit out of it. But then there is the problem of modesty. A person should not go to the bathroom where everyone can see him.
The Talmud offers two solutions. 1) The toilet will be for children, who do not need to be modest when they relieve themselves. 2) Instead of using the place for a toilet, he could put up a fence of thorns and shrubs. This will stop the idolatrous place from using that area and he will still not be providing a good wall for an idolatrous shrine.

If the shrine was created from the beginning to be idolatrous, it is totally prohibited from Jewish usage. If the building originally existed for other, non-idolatrous, purposes, and then was somehow modified to be idolatrous, the Jew needs to remove the renovations before it is permitted to use the building. In other words, the earlier structure is permitted and only the new parts that were created for idolatry are forbidden. If the idol was only brought into the house, the house is not actually “idolatrous.” The idol may be taken out and the house is permitted.

The Talmud analyzes the underpinnings of Rav’s statement that if a person worships a house, he turns it into an idol that is now prohibited to Jews. A house consists of pieces that were once detached from the ground and now are attached. Things attached to the ground cannot become prohibited if they are worshipped. So it seems like Rav says although the house is currently attached, since its parts were once attached, it is still prohibited.

The problem the Talmud has is that the Mishnah says that if he built the house for idolatry it is prohibited. By implication, if he built it and then worshipped it, it is not prohibited. In such a case, since it is already attached to the ground, it does not become prohibited.
The resolution is that the house becomes prohibited whether he built it for idolatry and then worshipped it or if he worshipped it but did not build it for idolatry. In either case, the house becomes prohibited.

To Rav there really should be four clauses in the mishnah: one where he built the house for idolatry, and another where he built it for other purposes and only later worshipped it. So why are there only three?
The answer is that when it comes to annulling the house as an idol, it does not matter whether it was built for idolatry or only worshipped after it was already built, the tanna of the Mishnah included both categories in one clause.

Ami refers to the second clause in the mishnah. For it to become prohibited he must plaster or cement inside the stone itself, not merely on its outside. It seems that what he is trying to say is that for something to be considered an idol, it must be an idol “on the inside.” Almost the same way we treat human beings—our identity is formed more by our inside, than by our external features.

Ami refers to the second clause in the mishnah. For it to become prohibited he must plaster or cement inside the stone itself, not merely on its outside. It seems that what he is trying to say is that for something to be considered an idol, it must be an idol “on the inside.” Almost the same way we treat human beings—our identity is formed more by our inside, than by our external features.

The Talmud raises a difficulty on R. Ami. In the mishnah, stones seem to be analogous with the house. Since when it comes to the house, the plastering was on the outside and yet it was prohibited, so too when it comes to the stones. Even though the plastering is on the outside, the stones should be prohibited.
The solution is that when it comes to the house, there is plastering on the inside too, between the bricks.
The problem with this solution is that the Mishnah might be dealing with a case where the house was first plastered to be a regular house, and then was again plastered for it to be an idolatrous shrine. Such a house would be prohibited even though the plastering was on the outside.

Ami was not referring to the forming of the stone but to its annulment. What he said was that even if the person had put the plaster or tile in the stone, if he removes it the stone goes back to being permitted (it is annulled). We might have thought that since the plaster was in the stone, it was like making the stone for idolatrous purposes, so R. Ami teaches us that it is not.