Both parashiyot this Shabbat deal with tumah and taharah (purity)th tumah and taharah (purity). But what is the meaning of these terms?

A talmudic tale tells of a non-Jew who asked Rabbi Yochanan ben Zakkai about the laws of purity, which seemed to the non-Jew to be nothing but witchcraft. Rabbi Yochanan attempted to give him an explanation in accordance with contemporaneous practice of medicine.
 
Later, however, Rabbi Yochanan’s students, believing he hadn’t given a truthful answer, press him for an honest response, to which Rabbi Yochanan replies: “By your lives, a dead person doesn’t make things impure, and the water doesn’t make things pure. Rather, God said, ‘I have engraved a rule, I have decreed a decree and you have no permission to transgress what I decreed.’”

This story captures two polarised conceptions as to the nature of tumah and taharah. The rabbi’s answer to the non-Jew reflects an understanding of them as something real, whether physical or metaphysical. His answer to his students, however, sees nothing there other than God’s commandment. 

These opposing understandings are reflected in the opinions of later medieval sages. For instance, Ramban, in his commentary to Deuteronomy 14:3, sees tumah as an actual metaphysical entity while Maimonides, in his Hilchot Mikvaot, 11:12, sees it as nothing but of symbolic significance. 

A new perspective was suggested by Mary Douglas in Purity and Danger. Some of the details might sound controversial to the traditional mind but the gist of her idea is refreshing nonetheless. Douglas suggests that tumah, as its equivalences in other cultures, is simply something which is misplaced. In fact, she states, this concept of misplaced stuff is true to all times and cultures. For instance, food on a plate is just fine. But once misplaced, for example on to the floor or one’s shirt, it turns into “dirt”. 

Far from trivialising the concept of tumah and taharah, Douglas is in fact emphasising its significance and importance in all aspects of our daily life.