פירוש על בבא בתרא 2:4

מי שהיה כותלו סמוך לכותל חבירו – in the form of a גא"ם/Gamma – an angle, and he comes to make a second wall opposite the wall of his fellow until he makes the three walls like the form of the [Hebrew letter] "בי"ת"/Bet, his fellow will prevent him, until he distances it from opposite him four cubits, in order that the place between the two walls will be wide and many will be able to tread there. For the walking of feet on the ground nearest the wall strengthens the foundations of the wall and preserves them. And especially the wall of a garden or he wall of a courtyard that is in a new city where many have not [yet] tread, and the ground was not strengthened as much as needed through treading of the feet [of people]. But, in the wall of a courtyard that is in an old city is supported and there is no need to distance it. And similarly, if the wall of his fellow was not four cubits or more, he doesn’t have to distance it, for a wall that is less than four cubits does not need strengthening.

Introduction Mishnah four discusses how far a person must distance his wall from his neighbor’s walls.

ובחלונות מלמעלן מלמטן ומכנגדן ד' אמות – he had a wall above his wall and his fellow built a wall opposite the window from below. If there remained from the top of the wall that he built up until the window less than four cubits in height, he forces him to lower the wall in order tha the not stand on the top of the wall and look out through the window.

If one’s wall was adjacent to his friend’s wall he may not build another wall adjoining it unless it is at a distance of four cubits. If a person already owns a wall adjacent to his friend’s wall, he may wish to add another wall, thereby creating a three walled structure. If he wishes to do that he must leave four cubits between the new wall and his friend’s wall in order that people will walk near his friend’s wall. It was believed that people walking near a wall would strengthen the foundations of the wall. If he didn’t leave enough room next to his friend’s wall people would not walk there, thereby depriving his friend of that benefit.

מלמטן – the window was lower than the wall, he forces his fellow to raise the wall that he built opposite him at the height of four cubits from the window in order that he should not see him.

And [if he builds a wall opposite his friend’s] windows, whether it is higher or lower than them or level with them, it may not be within four cubits. If a person wishes to build a wall opposite another person’s windows he must fulfill three conditions: 1) If the wall is higher than the window it must be more than four cubits higher so that he will not be able to use his wall to look into his friend’s window. 2) If the wall is lower than the window it must be more than four cubits lower so that he will not be able to stand on the wall and look into the window. 3) If it is across, it must be four cubits away so that it will not block the light going into the other window.

ומכנגד – he must distance the wall from the window four cubits in order that he will not obscure his light.

Questions for Further Thought:
• Why is the mishnah so concerned with people looking into other people’s windows? What does this teach us about privacy?