Chapter 1, Mishnah 4, which continues the discussion of acquiring ownerless objects (hefker). One who acquires something from ownerless property requires an actual act of acquisition (kinyan), an action that effects the transfer of ownership. For movable objects - mobile items - the standard act of acquisition is lifting (hagbahah) the object, whereas other actions are ineffective.
Additional methods of acquisition:
Acquisition by courtyard (kinyan chatzer) - When the object is placed in a person's private domain, and it is "guarded", meaning that the object rests there securely and will not disappear, since a fence or wall surrounds the domain and no one can reach it except by theft. In such a case, the courtyard is considered like the owner's hand and acquires it for him.
Acquisition by four cubits (kinyan daled amos) - It is a rabbinic enactment that one who stands within four cubits of an object acquires it by his very proximity to it. This enactment was designed to prevent quarrels among people: the first person to arrive at the location acquires the object, even without lifting it.
However, the rule that a person's four cubits acquire for him applies only in a place that is not a full public domain and is not the domain of another person. In the public domain, where many people are present, a person does not have his own defined and separate four cubits, and therefore the enactment does not apply there - for the very same quarrels would naturally arise from it anyway. This is not the case in an alleyway, on the sides of the road, or in a place without much traffic, where a person stands in his own separate space: there, his four cubits acquire for him, and there is no need for lifting at all.
The first clause - "Nafal alav":
A person walking in the public domain sees a found object lying on the ground - a perutah, a dollar, or a million dollars, and "venafal alav" - he fell upon it, prostrating himself over it in order to acquire it. Another person comes along and "vehichzik bah" - took hold of it. The term "takes hold" usually means merely grasping, but in the context of the Mishnah, it does not mean mere grasping, but rather that he grasped and lifted the object from the ground. Through this, the object becomes his, and the one who lifts it is the one who acquires it.
Even though the first person is lying on the found object, it is meaningless: jumping on the object is not an act of acquisition, and a person does not acquire by sitting or prostrating himself upon it. Furthermore, since this takes place in the public domain, even the first person's prior presence within four cubits does not help him.
Analysis: Falling upon an object not in the public domain:
What is the law in a place that is not the public domain - the sides of a road without traffic, an alleyway, a private path, and the like? Many authorities learn that in such a situation, a person's four cubits would acquire the object for him, and the Mishnah is not dealing with this case at all. However, there are those who say that the very fact that the person jumped on the found object and prostrated himself over it reveals that his intention was to acquire it specifically in this manner, by sitting or lying on it, and not through the mechanism of four cubits. And since he demonstrated his intention not to acquire it via four cubits, he did not automatically acquire it. According to this approach, it turns out that someone who snatches the object from underneath the first person's body and lifts it - has acquired it and is permitted to keep it.
The latter clause - A guarded courtyard regarding animals:
Although the rule is that a person's guarded courtyard acquires for him, this rule generally does not apply to animals: as long as the animal can enter and exit, jump over the fence, and the like, the courtyard is not considered guarded with respect to it. However, if the animal's leg is broken and it is unable to do so, and the owner of the courtyard is standing nearby and can reach it before it leaves - the courtyard is considered guarded relative to that animal and acquires it for him.
Therefore, an owner of a courtyard who sees people running after a found object - after a deer whose leg is broken and cannot run away, or after fledglings that cannot yet fly - and says, "Zachatah li sadi" - my field has acquired it for me, then "zachatah lo" - it has acquired it for him. This is not referring to someone who hears about this from afar, but rather to someone who is standing close by and can approach and catch the deer or the fledgling himself. In such a case, it is considered "guarded" in his courtyard, since the animal cannot flee from there, whether due to its broken leg or because the fledglings cannot fly.
In contrast, "hayah tzvi ratz k'darko o shehayu gozlos mafrichin" - if a deer was running normally, leaping in a person's courtyard such that it would jump over even a high fence, or if there were fledglings within his fenced-in area that can fly away from there - "v'amar zachasah li sadi" - and he said, "My field has acquired it for me," meaning his desire is that his field and property should acquire the animal for him - "lo amar klum" - he has said nothing. His statement has no substance, since the item is not secured: the bird will fly away and the deer will leap out. The animal therefore remains ownerless, and anyone who catches it and lifts it - acquires it for himself.
Is an actual verbal statement required?
When a person is standing nearby, and the fledgling cannot fly - must he explicitly say that the courtyard acquires it for him, or is thought sufficient, since the field acts as his hand and acquires it for him when it is secured?
The Rambam - The Mishnah means what it says and says what it means. Therefore, one must verbalize the words and say that the field acquires it for him, and if he did not say so - he has not acquired the animal.
Tosafos and the Rosh - There is no absolute need to say this explicitly, and thought alone is sufficient. If the field is his and it is secured relative to where he is standing, it acquires it for him.
As a matter of halachah, the Mechaber rules in the Shulchan Aruch in accordance with the opinion of the Rambam, that one must say this explicitly in order to acquire it. The Rema, however, rules like Tosafos and the Rosh, that even thought is sufficient for the field to acquire the animal for him, and he can then catch it himself and keep it.
According to this approach, the reason the Mishnah states that the person declares, "My field has acquired it for me," is only so that those running to catch the deer with the broken leg will stop running, so that they do not come to a dispute, as he is informing them that he has already preceded them and acquired it.
In summary: In this Mishnah we learned that one who falls upon a found object has not made an act of acquisition, and the one who lifts it up after him is the one who acquires it, since in the public domain, even one's four amos do not acquire for him. We also learned that a person's courtyard acquires animals for him only when it is secured concerning them - a broken deer and fledglings that have not flown while he is standing nearby - whereas regarding a deer running normally and fledglings that are flying away, "he has said nothing." Finally, we noted the dispute between the Rambam on one hand, and Tosafos and the Rosh on the other, whether an explicit statement is required or if thought is sufficient, and the rulings of the Mechaber and the Rema.