Eruvin, Chapter 6, Mishnah 9. This mishnah deals with two courtyards, one inside the other, connected by a doorway between them, where the inner courtyard has a right of passage through the outer one - that is, its route out to the public domain passes through the outer courtyard. The question is whether the status of the inner courtyard affects the permissibility of carrying in the outer courtyard.
Three opinions are presented in the mishnah:
The opinion of the first tanna - a foot that is permitted in its own place does not create a prohibition: If the inner courtyard made an eruv for itself, so that its residents may carry from their houses into their courtyard, its right of passage does not create a prohibition in the outer courtyard. 'Foot' - the ability of the residents of the inner courtyard to pass through the outer courtyard; 'permitted in its own place' - that it has an eruv of its own. But if it were prohibited in its own place, such as when they did not make an eruv in the inner courtyard, this would create a prohibition for the outer courtyard.
The opinion of Rabbi Akiva - even a foot that is permitted in its own place creates a prohibition: Even when they made an eruv in the inner courtyard so that it is permitted within its own domain, the very fact that it has passage into the outer courtyard and the two did not make an eruv together renders the outer courtyard prohibited.
The final opinion - even a foot that is prohibited in its own place does not create a prohibition: Even if the inner courtyard did not make an eruv, this is not enough to render the outer courtyard prohibited.
Let us look at the wording of the mishnah:
"Eirvah hapenimis velo hachitzonah" - the inner courtyard is permitted to carry from the houses into the courtyard, and the outer courtyard is prohibited. This prohibition does not stem from the inner courtyard, but from the fact that the residents of the outer courtyard themselves did not make an eruv among themselves.
"Eirvah hachitzonah velo hapenimis" - both are prohibited. The inner courtyard is prohibited because it has no eruv; and the outer courtyard, even though it made an eruv for itself, becomes prohibited because the inner courtyard is prohibited in its own place and has a right of passage through the outer one, and therefore it renders it prohibited.
"Eirvu zu le'atzmah vezu le'atzmah" - each of the two courtyards made an eruv on its own, but they did not make an eruv together.
At this point the opinions diverge:
According to the first tanna, each one is permitted on its own. This is the expression of the principle of 'a foot that is permitted in its own place': even though the inner courtyard has a right of passage through the outer one, since it is permitted within its own domain, it does not render the outer courtyard prohibited.
Rabbi Akiva disagrees and says that "hachitzonah asurah... shedrisas haregel osartah" - even though the inner courtyard made an eruv for itself and the outer courtyard made an eruv for itself, the very ability of the inner courtyard to pass through the outer courtyard, combined with the fact that the two did not make an eruv together, renders the outer courtyard prohibited.
And Rabbi Yosi says that the right of passage - the possibility of passing from the inner courtyard through the outer one - does not adversely affect the outer courtyard. Therefore, if the outer courtyard made an eruv, even if the inner one did not, this does not render it prohibited.
In summary: in this mishnah we learned three approaches to the law of two courtyards, one inside the other: according to the first tanna, a foot that is permitted in its own place does not create a prohibition elsewhere, while a foot that is prohibited in its own place does create a prohibition; according to Rabbi Akiva, even a foot that is permitted in its own place renders the outer courtyard prohibited; and according to Rabbi Yosi, even a foot that is prohibited in its own place does not render the outer courtyard prohibited at all.