Kelim, Chapter Eighteen, Mishnah Eight
The tefillin box worn on one’s head has four compartments to hold four scrolls. The box was defiled with corpse impurity. Our mishnah deals with cases where one replaces the compartments, eventually causing the entire tefillin box to be pure.
1) A tefillin box consisting of four vessels: if the first compartment was unloosed, and then he mended it, it retains its corpse uncleanness.
a) So is it also the case with the second and the third.
2) If he unloosed the fourth it becomes free from corpse uncleanness but it is still unclean from contact with corpse uncleanness.
3) If he went back to the first compartment and unloosed and mended it, it remains unclean from contact.
a) So also in the case of the second compartment.
4) If he then unloosed the third compartment and mended it becomes clean, since the fourth is unclean from contact, and what is unclean from contact cannot convey uncleanness by contact.
Section one: If he opens up the first compartment and removes it, and then makes a new compartment, the whole box of tefillin retains its corpse impurity. The same is true if he removes and then repairs all three of the first boxes. Since one compartment is still there, the tefillin is not considered new and it retains the same level of impurity.
Section two: If after having replaced all three of the first compartments, he now replaces the fourth compartment, then the box loses it corpse impurity because it is an entirely new tefillin box. However, the box still has some level of uncleanness. When he replaced each of the first three compartments he put them back into contact with a compartment that has corpse uncleanness. The unclean compartment which is a “father of impurity” defiles by contact the compartment that it touches, giving it first degree impurity. So all of the first three compartments have this status, and therefore the whole box has this status.
Section three: Going back and undoing the first compartment and then replacing it, and then doing the same with the second department, will not cause the box to be entirely clean. Because some of the compartments are unclean, the whole box is unclean.
Section four: However, if he goes back and replaces the third compartment, the whole box becomes completely pure. As long as one of the first three replacement boxes was still around, the whole box is impure with first degree impurity, because these three replacement compartments came into contact with something that was a “father of impurity.” The first came into contact with the second, the second witht the third and the third with the fourth. However, once the first three compartments are replaced again, none of the compartments left was ever in contact with something that was a “father of impurity.” All that is left is the fourth compartment which also was never in contact with a “father of impurity.” It has “first degree impurity” only because it was part of a box that had “first degree impurity.” The fourth compartment does not now return to defile the new third compartment, because something with “first degree impurity” does not defile other vessels. And since the first three compartments are completely pure, the fourth one is also pure.