Somebody whose house was next to a place of Avodah Zarah (idol worship), where they worshipped, had a shared wall between his house and the place of worship. If that wall fell down, he was not allowed to rebuild it in the same spot, because even though he would be rebuilding it for his own home, he would technically be building a wall for idol worship. What should he do to rebuild the wall so he can actually live in the house? He should go into his own property, moving away from the place of Avodah Zarah at least four Amot (cubits), because within four Amot there is some sort of protection. Then he can build a wall there that will serve him alone and will not be for the idol-worship house.
The land where the wall fell, where the wall had stood, was partly his and partly the idol’s. Let’s say he was a 50-50 partner in it. He calculates half of the land under as his, and half as the Avodah Zarah’s. So the Avodah Zarah’s portion would not count toward his four Amot, but the Jews’ half would. For example, let’s say the wall itself was two Amot wide, and he owned half of it. What he could count toward the four Amot he is trying to be away from the Avodah Zarah is one of those Amot plus three more, for a distance of four. That would be something he could do in order to build a new wall four Amot away from the Avodah Zarah.
The stones, the wood, and the sand transmit Tum’ah (ritual impurity) like a Sheretz (creeping thing). Like the Pasuk (verse) says, “you should treat it with disgust,” speaking about the Avodah Zarah. So the Avodah Zarah transmits Tum’ah like a Sheretz. A Sheretz transmits Tum’ah through touch, so an idol does as well. But a Sheretz only transmits through touch—touching the actual idol—not to somebody who carries it without actually touching it. Let’s say they carry it with something in between. That would not transmit Tum’ah. According to the previous opinion, an idol is compared to a Sheretz, and therefore it does not transmit Tum’ah to somebody who carries it without directly touching it.
We’ll bring another, more stringent opinion. Rabbi Akiva says, ke-nidah—like a Nidah (menstruant): the stone, wood, and sand of Avodah Zarah transmit Tum’ah like a Nidah. Shene’emar (as it is said): tizrem kemo davah, tze tomar lo. Like the Pasuk in Yeshayah says, “You will scatter them like a distanced Nidah. You will tell it leave.” So we compare the Avodah Zarah to a Nidah. Ma Nidah metame’ah be’masa, af Avodah Zarah metame’ah be’masa. This comparison between a Nidah and an idol tells us that just like a Nidah transmits Tum’ah when it’s actually even carried, the same thing applies to Avodah Zarah. The Nidah is like an Avodah Zarah. The idol also transmits Tum’ah when it’s carried, even without touching it.
These opinions, both of them, are not biblical, but it’s asmachta be’alma. It’s a rabbinic type of Tum’ah, and the rabbis are using the text of Pesukim (verses) to support the law they’re creating at the Rabbanan (the rabbis) level.