פירוש על כלים 25:5

הרובע ואחוריו טמאים – the quarter – that is the inside. And the backsides are the walls that surround the quarter from outside. But the wall that forms a partition between both of them, is included in the inside of the half-quarter, and it is pure to the side that is towards the one-half quarter. And similarly, when the one-half quarter is defiled, the half-quarter and its outsides are impure. The half-quarter is the insides, and the outsides are the walls that surround that half-quarter from the outside, but the wall that forms a partition, the side that is towards the quarter is included with the inside of the quarter.

Introduction This mishnah continues to discuss the measuring vessel that has two sides one of a quarter log and one of a half-quarter.

נטמאו אחורי הרובע – that is, the walls that surround from the outside.

If the [inside of the] quarter contracted uncleanness, the quarter and its outer side are unclean, but the half quarter and its outer side remain clean. Having determined in yesterday's mishnah that each vessel is independent, the mishnah now deals with the thorny issue of their shared wall. If the inside of the quarter log became unclean, its own outer side is also unclean. However, the walls of the vessel that surround the half-quarter measuring cup remain clean.

אין חולקין את הגביים (they do not divide the hind parts/outside parts [of the utensil] – that is the outside parts, for all the outside parts are impure when one of the outside parts became impure.

If the [inside of the] half quarter contracted uncleanness, the half quarter and its outer side are unclean, but the quarter and its outer side remain clean. The same rules apply if the inside of the half-quarter vessel become unclean.

מטביל את כולו – the quarter and one-half of the quarter. But the Halakha is not according to Rabbi Meir. And the law of the middle wall is explained above at the end of Chapter Two (Tractate Kelim, Chapter 2, Mishnah 7) for “Rabbi Yohanan ben Nuri says: They divide its thickness. The [half that] serves the unclean [portion] is unclean; the [half that] serves the clean [part] is clean.”

If the outer side of the quarter contracted uncleanness,the outer side of the half quarter remains clean, the words of Rabbi Meir. But the sages say: the outer side cannot be divided. According to Rabbi Meir, if the outer wall of the quarter vessel becomes unclean, only this outer wall is unclean. The outer wall of the half-quarter remains clean. In other words, although this wall surrounds the entire vessel, we look at it as if it were divided in half and only the part that surrounds the quarter is unclean. The other sages disagree and say that you can't divide the wall of a vessel in half. So if the outer wall of either side becomes unclean, both outer walls are considered unclean.

When he immerses the vessel, he must immerse the whole thing. Despite the fact that it is possible for one side to be clean and the other to be unclean, when he immerses the vessel, he must immerse both sides. If he does not do so, the vessel remains unclean. Note that this is similar to the end of mishnah three.