Commentary for Bava Batra 4:12

And he who asked it, why did he ask it: And did he not know that fell is different?

And he who asked it, why did he ask it: Meaning to say it is obvious that fell is different. So he should have, just the opposite, shown from that which it teaches, "and if it fell," that in general, it is not considered damage.

It was necessary [to teach] the latter clause: Meaning to say, the law is also the same from the start (before it fell). So that which it taught, "If it fell," is because the latter clause makes us understand that we do not obligate [him to rebuild] more than four ells. And even though the first [wall] was higher, we do not obligate him.