Chagigah 21 - How is one level of purity different from another?
We mentioned that there are five levels of purity , each one higher than the other. However, this is not mentioned anywhere in the Torah, where a food or a person can by either ritually pure or not. (Even that is not practiced today, when there is no Temple and no ashes of red heifer).
The Sages though instituted those multiple levels of purity, in order to train people in guarding objects from impurity. Holy (sacrifice-related) objects and the priest's portion (terumah) represent the fourth and the third levels of purity. There are ten areas where the purity of holy objects is stricter than those of priest's portion.
For example, when one takes objects to a mikveh to purify them by submersing in its water, he can dip them with one object being inside the other - so long as the water enters into all areas of the object; but this is allowed for priest's portion but not for holy objects. Why? It may happen that one of the objects, the one which is inside the other, is too heavy, and the place of their contact does not admit water. That is not a serious concern though, and the Sages added this only for holy objects.
Art: A personification of Purity by Simone Pignone