Maseches Ma'asros, chapter three, mishnah 5. We have learned that when a person intends to bring his produce home, the completion of the work - finishing the processing of the produce in the field - is not enough to trigger the absolute obligation of tithing that forbids even a casual snack; he must first bring the produce into his house. We also learned that once the produce reaches his courtyard, assuming this is the place he normally brings it to on his way home, the courtyard too establishes the obligation of tithing.
But not every courtyard creates an obligation to separate tithes. The courtyard has to function as the person's home: if it has the essential features of a house, then it is his house in every respect - an essential house, as it were, only without a roof. Our mishnah therefore discusses what exactly constitutes the essential feature of a house, the one that turns a courtyard into a courtyard that obligates the separation of tithes.
Four opinions are presented here, and they divide into two categories: is the essential feature of a house the security it provides, or the privacy it affords?
The mishnah asks: "Eizo chatzer shechayeves b'ma'asros":
Which courtyard obligates the taking of tithes absolutely, even before a casual snack? The first two opinions focus on the aspect of security:
"Rabbi Yishmael omer: chatzer haTzuris" - a courtyard like the courtyards in the city of Tzur in Lebanon, which has a watchman standing at its entrance. "Shehakeilim nishmarim b'sochah" - if a person leaves his belongings in such a courtyard, they are protected, because the watchman guards them. In Rabbi Yishmael's view, the essential feature of a house is the security it provides: you can leave things there without worrying that they will be taken. A courtyard that has a comparable feature is likewise obligated in the taking of tithes.
"Rabbi Akiva omer: kol she'echad pose'ach v'echad no'el, peturah" - in his view the essential feature is that the person, and he alone, decides whether the doors are locked or open. When two households live in a courtyard, each resident has his own key, and the fact that Mr. A locked the courtyard door at night does not prevent Mr. B from opening it. Since a person cannot be certain that he is the one who determines when the doors are locked and when they are open, this is not a courtyard with the status of a house, and such a courtyard is exempt from the obligations of tithing.
Rabbi Nechemiah and the Tanna who follows him focus mainly on the angle of privacy:
"Rabbi Nechemiah omer: kol she'ein adam bosh mile'echol b'sochah, chayeves" - any courtyard that affords enough privacy that a person can attend there to private matters, the sort of thing one does not do in public view. As we have mentioned, in the time of the Mishnah people did not eat in the public domain, only in the private domain. If a person is not embarrassed to eat in his courtyard, meaning that he feels there is sufficient privacy there, such a courtyard obligates the separation of tithes.
"Rabbi Yosei omer: kol shenichnas lah v'ein omer mah atah mevakesh, peturah" - a courtyard that does not have enough privacy, where someone who walks in is not confronted by anyone asking him "what do you want" - what is your business here. Since the courtyard lacks the level of privacy that would allow one to ask people who enter what they want in a place where they do not belong, it is not like a house, and its owner is therefore exempt from tithes.
In practice the halachah takes all four of these opinions stringently: since we have no clear ruling as to whose view is authoritative, any courtyard that has one of these four features - food that enters it intentionally becomes obligated in tithes.
Rabbi Yehudah's addition:
Rabbi Yehudah holds like Rabbi Akiva, that a courtyard whose doors the person is not the only one entitled to lock is not a private courtyard, and he adds a special case: a courtyard that has an inner resident and an outer resident, where the inner resident has to pass through the outer resident's property in order to get out. In this case the inner courtyard belongs to the inner resident alone, since he alone holds its key, and it therefore obligates the separation of tithes. But the outer courtyard, to which both residents - the inner and the outer - hold keys, is exempt from the obligation to separate tithes before a casual snack.
In the words of the mishnah: "Rabbi Yehudah omer: shnei chatzeros zu lifnim mizu" - two courtyards, one inside the other, "hapenimis chayeves" - the inner one is obligated in the separation of tithes, "v'hachitzonah peturah" - while the outer one remains exempt from the separation of tithes.
In summary: In this Mishnah we examined the question of which courtyard is considered like a house and therefore obligates produce in tithes. Four opinions were stated, which divide into two underlying principles: the principle of security - a Tyrian courtyard, in which vessels are kept safe (Rabbi Yishmael), and a courtyard where it is not the case that one person opens it and another locks it (Rabbi Akiva); and the principle of privacy - a courtyard in which a person is not embarrassed to eat (Rabbi Nechemyah), and a courtyard where someone who enters is asked "What are you looking for?" (Rabbi Yosei). In practice we follow all four opinions stringently. Finally, we learned Rabbi Yehudah's addition regarding two courtyards, one inside the other, that the inner one obligates and the outer one is exempt.