Ohalot, Chapter Thirteen, Mishnah Three
Today’s mishnah deals with how large an opening in a door must be to allow in and out impurity.
1) [With regard to] a hole in the door:
a) Its minimum size is that of a fist, the words of Rabbi Akiva.
b) Rabbi Tarfon says: one handbreadth square.
2) If the carpenter had left a space at the bottom or the top [of the door] or if one had shut [the door] but not closed it tightly, or if the wind blew it open, the minimum size is that of a fist.
Section one: Rabbi Akiva holds that since these holes in the door were not made intentionally, they should be compared with the naturally forming holes referred to in mishnah one. For impurity to travel through such a hole, it must be the size of a fist.
Rabbi Tarfon says that it needs only be a handbreadth square (slightly smaller than a fist) for these holes are meant for human use (see mishnah one).
Section two: There are three spaces in a door referred to here: 1) the carpenter didn’t make the door as large as it was supposed to be; 2) the door wasn’t tightly closed; 3) the wind blew the door slightly open. All of these are openings that were probably not intentionally formed, nor are they holes that are used by human. Therefore, the measure to allow through impurity is that of a fist, which is larger than a handbreadth.