Rava replied: From the following verse, "You shall live in booths for seven days" (Leviticus 23:42) the Torah declared: For seven days leave your permanent dwelling place and dwell in a temporary dwelling place. Up to twenty cubits [high] one makes his home a temporary one; higher than twenty cubits, one does not make his home temporary, but permanent. Abaye said to him: If so, if he made walls of iron and placed the [proper] skhakh over them, would it also not be a valid Sukkah. He answered him: This is what I mean to tell you: Up to twenty cubits, which a man makes his temporary home, even if he makes it permanent,he has fulfilled his mitzvah; [but in one] higher than twenty cubits, such as a man makes his permanent home, even if he makes it temporary, he has not fulfilled his obligation.