Eruvin, Chapter 4, Mishnah 9. In the previous Mishnah we learned how a person who identifies an object almost two thousand amos away as Shabbos is about to begin can declare that his place of rest will be at that object, and use it as the center point of his techum. Our Mishnah comments on this: this case, in which a person identifies an object and says "shevisasi sham" - "my place of rest is there" - is precisely what the Sages meant when they said that a poor man "me'arev b'raglav" - makes his eruv with his feet.
Two options are open to him, and he does not need bread or any other food in order to make the eruv:
"Shevisasi kan bamakom she'ani omed" - "my place of rest is here, where I am standing." He makes an eruv by virtue of being physically present there with his feet.
"Shevisasi sham" - "my place of rest is there." He identifies a spot ahead of him and establishes his place of rest there.
But this leniency was stated only with regard to a poor man.
What is a 'poor man' in this context:
This does not necessarily mean someone who has no money, but rather someone who has no food with him, that is, someone who has no food available with which to make an eruv. A very wealthy man who is traveling and has no food with him is considered a 'poor man' for this purpose, and he may make his eruv with his feet or by designating a particular spot.
The dispute between Rabbi and Rabbi Yehudah:
Rabbi: The primary way of making an eruv is with bread. That is how the Sages instituted the rabbinic requirement of eruvin from the outset - that it be done with bread. For someone who has no bread with him, whom they called a 'poor man,' they made a special leniency allowing him to make his eruv with his feet.
Rabbi Yehudah: Both the one who has bread and the one who does not have bread - both of them make their eruv with their feet. The primary way of making an eruv is through physical presence at the spot or by designating a spot, and not by using bread.
What, then, is the leniency granted to a wealthy man, that is, to someone who does have bread available? That he is not required to go out himself and make the eruv by being present at bein hashemashos. He may send a messenger, or do it himself earlier in the day, and he need not be there physically at the time of bein hashemashos. But the primary way, according to Rabbi Yehudah, is with one's feet.
The limitations that even Rabbi Yehudah concedes:
The Gemara explains that even though in Rabbi Yehudah's view someone who has bread available may go out and establish his place of rest by being there at bein hashemashos, there are two limitations that even he concedes:
One who is sitting at home: He cannot declare that his place of rest will be at a certain location two thousand amos from his house. To walk there - he is permitted; but to declare it he cannot. Only someone who is out on the road, as the previous Mishnah said, may look at an object located almost two thousand amos away and declare that that spot will be his place of rest. This leniency stems from the fact that he is on the road, and this is so even according to Rabbi Yehudah.
The condition of being able to reach the spot: Even according to Rabbi Yehudah, one who designates an object located slightly less than two thousand amos away and wants his place of rest to be there - his declaration is effective only if he could have reached it by running before Shabbos began. Even though he is tired or lazy and does not actually want to run, he must be theoretically capable of reaching the spot he designated before Shabbos actually begins, and only then is the declaration valid.
In summary: In this Mishnah we learned that the case of the previous Mishnah - establishing one's place of rest at an object located within two thousand amos - is itself what the Sages meant when they said that a poor man 'makes his eruv with his feet,' whether by establishing his place of rest where he is standing or by designating a spot ahead of him. A 'poor man' in this context is someone who has no food with him for making an eruv, even if he is in fact wealthy. Rabbi and Rabbi Yehudah disagree about the essence of the enactment of eruv: according to Rabbi it is with bread, and making an eruv with one's feet is a leniency for someone who lacks bread; according to Rabbi Yehudah it is with one's feet, and the leniency for the one who has bread is that he is not required to be present at bein hashemashos but may send a messenger or do it in advance. And even Rabbi Yehudah concedes that one who is sitting at home cannot establish a place of rest from a distance, and that the declaration is valid only if he could have reached the spot by running before Shabbos began.