Kelim, Chapter Twenty-Four, Mishnah One
Every mishnah in Chapter Twenty-Four of Kelim has the format of “there are three different types of X.” The differences between the different types of X (in our case shields) means that there are differences as to their susceptibility to impurity. All of the mishnayot have the same structure. The first item is susceptible to midras uncleanness because it can be used as a seat. The second item is not used for sitting but it is a vessel so it is susceptible to corpse uncleanness. The third item is completely pure.
There are three different types of shields:
1) A bent shield is susceptible to midras uncleanness;
2) A shield used in the arena is susceptible to corpse uncleanness;
3) And the Arabian toy shield is pure from all uncleanness.
Section one: A bent shield can be used to lie upon and therefore it is susceptible even to midras uncleanness. All the more so it is susceptible to other types of uncleanness.
Section two: A shield used in battle is not meant to be laid upon and therefore it is not susceptible to midras uncleanness. However, like all vessels it is subject to corpse uncleanness, meaning if it comes into contact with a corpse, it is unclean.
Section three: A shield that is not used as protection is not susceptible to impurity. This is the case with the Arabian toy shield.