Rambam - 1 Chapter a Day

When a person left his city on Friday and stood in a specific place within the Sabbath limits, or at the end of the Sabbath limits, and said,1 "This is my place for the Sabbath," although he returns to his city and spends the night there, on the following day he is permitted to walk two thousand cubits from that place in every direction.

This is the principal manner [of establishing] an eruv t'chumin - actually to go there by foot.2 [The Sages allowed] one to establish an eruv by depositing an amount of food sufficient for two meals in the place - although one did not actually go there and stand there - to expedite matters for a rich person, so that he will not have to travel by himself, and could instead send his eruv with an agent who will deposit it for him.

Similarly, when a person decides to establish his place for the Sabbath in a specific location - e.g., at a tree, a house, or a fence that he can identify, and at nightfall:

a) there are two thousand cubits or less between him and that place; and

b) he sets out to reach that place and establishes it to be his place for the Sabbath,

on the following day, he may proceed to that desired location and continue two thousand cubits in all directions. [Moreover, this law applies] even when he did not actually reach that place or stand there, but instead a friend had him turn back and spend the night at his home, or even if he himself decided to turn back, or was prevented [from going there by other factors].

[The rationale is that] since he made a resolve to establish [that location] as his place for the Sabbath, and set out for that purpose, it is considered as if he stood there or deposited his eruv there.

When does the above apply? To a poor person, for we do not burden him [with the obligation of] depositing an eruv, or to a person who is in a distant place - e.g., a person on a journey who is afraid that soon night will fall. [This leniency is granted] provided there is enough time in the day for him to reach the designated place before nightfall - if he ran with all of his strength - and there are two thousand cubits or less between him and that place at nightfall.3

If, however, he was not far from the place in question,4 nor was he a poor man, or

there was not enough time in the day for him to reach the designated place before nightfall if he ran with all of his strength, or

there were more than two thousand cubits between him and that place at nightfall, or

he did not specify the location he intended as his place for the Sabbath,5

he is not able to designate a distant location as his place for the Sabbath. Instead, he is granted no more than two thousand cubits in all directions from the place at which he is standing at nightfall.6

When a person stood in a private domain before the commencement of the Sabbath and designated it as his "place" for the Sabbath, or if he was traveling on a journey and had the intent of spending the Sabbath in a private domain that he knew and [thus] designated as his "place" for the Sabbath, he is entitled to walk throughout that domain and [continue] two thousand cubits in all directions.

If, however, this private domain was not enclosed for the purpose of habitation, or was a mound or a valley [different rules apply]: If it is the size of the area necessary to grow two se'ah [of grain] or less,7 one is entitled to walk throughout that domain and two thousand cubits in all directions.8 If it is larger than the size of the area necessary to grow two se'ah [of grain],9 [one's "place"] is considered to be only four cubits in that domain, [and one may proceed only] two thousand cubits from [this place] in all directions.

The same [law applies] when one places one's eruv in a domain that was not enclosed for the purpose of habitation.10

When a person [desires to] establish a distant location as his "place" for the Sabbath, but does not specify its exact location, he is not considered to have established it as his "place."

What is implied? A person was traveling on a journey and declared, "I will spend the Sabbath in such and such a place," "...in such and such a field," "...in such and such a valley," or "...a thousand cubits..." or "...two thousand cubits away from my present place," he has not established the distant location as his place for the Sabbath. [Instead,] he is entitled to proceed only two thousand cubits in all directions from the place where he is standing at nightfall.11

[The following rule applies] when a person says, "I will spend the Sabbath under this and this tree," or "...under this and this rock." If there are eight cubits or more under the tree or the rock, the person has not established [the location] as his "place" for the Sabbath, because he did not specify an exact location. For were he to spend the Sabbath in a particular four cubits, [he could be in error,] lest the other four cubits be the ones defined as his "Sabbath place."12

Therefore, it is necessary for a person to have the intent of establishing [a specific portion of the space - e.g.,] at [the tree's] base, its southern side, or its northern side, as his "Sabbath place."

If there are less than eight cubits under [the tree], and he intends to spend the Sabbath under it, he acquires it [as his "Sabbath place"]. [The rationale is that] there is no room [under the tree] for two places, and at least a portion of his [Sabbath] "place" has been defined.13

[This is the course of action to follow when] two people were coming on a journey, and one is familiar with a tree, fence, or other place that he desires to establish as his place, and the other is not familiar with the place. The person who is unfamiliar [with the place] should entrust the right to establish his "Sabbath place" to the one who is familiar with the place, and the latter should have the intent that he and his colleague should spend the Sabbath in the place with which he is familiar.

[The following rules apply when] the inhabitants of a city have sent a person to deposit their eruv in a specific place, he set out on his way, but a colleague had him return, and he did not deposit the eruv [on behalf of the inhabitants]: Since their eruv was not deposited in the desired location, [that location] is not established as their "Sabbath place," and they are not allowed to walk more than two thousand cubits in all directions from [the boundary of] their city.

[The person who went to deposit the eruv], by contrast, is considered to have established that location as his "place" for the Sabbath, because he had set out on the way to that location with the intent of establishing it as his "Sabbath place."14 Therefore, on the following day, he is permitted to proceed to the [desired] place and continue two thousand cubits from it in all directions.

The statement made previously,15 that a person who desires to establish a location as his Sabbath place from a distance need merely set out on the way, does not mean that he must depart and begin walking through the fields. Even if he merely descended from the loft with the intent of proceeding to [the desired] place, and before he left the entrance of his courtyard, a colleague prevailed on him to return, he is considered to have set out [on his way], and may establish his "Sabbath place" in that location.16

When a person establishes a location as his "Sabbath place" from a distance, he need not make an explicit statement, "This and this location is my 'Sabbath place.' It is sufficient for him to make a resolve within his heart and to set out on the way [to] establish that location as his "Sabbath place."17

Needless to say, a person who traveled by foot and actually stood at the location that [he desired to] establish as his "Sabbath place" need not make a statement. Making a resolve within his heart is sufficient to establish [the location as his "Sabbath place"].

When students who sleep in the house of study, but go and eat their Sabbath meals [in the homes of] people who live in the fields and the vineyards who show hospitality to wayfarers passing through, [the house of study is considered their "Sabbath place"] and not the place where they eat.18

They may walk two thousand cubits from the house of study in all directions. [The rationale is] that were it possible for them to eat in the house of study, they would not go to the fields at all. They consider the house of study alone as their dwelling.