As we learned on yesterday’s daf when dealing with a ḥazaka that serves as an act of acquisition, then the ḥazaka of na’al, gadar u-faratz will be effective. That is to say, if the recipient of the land locked the door, put up a fence, or performed any other action that exhibited ownership, such a ḥazaka will effect ownership.
Rav Nahman quotes Rabba bar Avuh on today’s daf as teaching that if someone builds a significant structure (a palterin, or castle), on land owned by a deceased convert who has no heirs – i.e. ownerless land – but someone else comes and puts doors into the structure, it is the second person who has successfully claimed ownership. He, after all, is the one who has performed the necessary na’al, gadar u-faratz. The Gemara explains that we see the act of the first person as merely being “moving around the bricks.” Since the house is not yet complete, the act of putting bricks together, and even completing walls, is not sufficiently significant to create a ḥazaka.
The Ri”d explains that since he has not performed the necessary na’al, gadar u-faratz, he has not successfully laid claim to the land. Therefore it is as though he has built on his friend’s property without permission, which does not give him any rights to the land. The Ramah argues that this must be a case where the bricks did not belong to the first builder – perhaps he found them on the field. Were they his own bricks, then when the second person comes and places a door in the unfinished house, he is placing it on someone else’s property, and the first person can remove the door and replace it with his own – thereby claiming the field. Tosafot explain that even though an almost completed building can serve a valuable purpose, e.g. as protection from the rain, as long as there are no doors it cannot be considered a dwelling place, which allows us to view it merely as a structure that has ruined the farmland or pasture. It will not be viewed as a constructive edifice – which would allow him to lay claim to the property – until it is complete.