Kelim, Chapter Twenty-Five, Mishnah Four
1) If [in a measure consisting of] a quarter [of a log] and half a quarter [of a log] the quarter measure contracted uncleanness, the half-quarter measure does not become unclean, and if the half-quarter contracted uncleanness the quarter does not become unclean.
2) They argued before Rabbi Akiva: since the half quarter measure is the outer part of the quarter measure, should not the outer side of the vessel whose inner side contracted uncleanness become unclean?
3) He answered them: Are you sure that it belongs to the category [of vessels] that have inner [and outer] parts? Perhaps the quarter is to be regarded as the outer side of the half quarter and, surely, the inner side of a vessel does not become unclean if the outer side contracted uncleanness.
Section one: Today’s mishnah deals with a measuring vessel that has two measures built into one vesselone that measures a quarter-log of liquid (about 1/2 a liter) and the other that measures a half-quarter of a log (about 250 ml). There is a partition that separates the two sides of the vessel.
According to the first opinion, which later will be identified with Rabbi Akiva, both sides are basically considered to be independent vessels. If one measure becomes unclean, the other measure remains clean. In other words, one vessel is not the inside vessel and the other the outside.
Section two: Some other figures, whose names are not mentioned, argue in front of Rabbi Akiva. These are probably students, as can be sensed from the way that Rabbi Akiva addresses them. In any case, they argue that since the main measuring cup is the half-quarter log, it should be considered the “inner” vessel. So if the inner, half-quarter log is defiled, the outer measure, the quarter-log, should also be considered impure.
Section three: Rabbi Akiva responds by noting that their assumption that the half-quarter is the inner vessel may not be correct. Rather, the half-quarter might be the outer vessel, in which case the other side is pure. Therefore, we have to treat each side as if it might be the outside, in which case no matter which side is defiled, the other side remains pure.