Here it is argued that by placing the redaction of Leviticus and Numbers firmly in the period in which they are supposed to have been edited, that is in the context of the Second Temple community, some problems of interpretation can be resolved. From the admittedly dubious memoirs of Ezra and Nehemiah three elements seem to be reliable: there was at that point a strong zenophobia on the part of the repatriates, a problem about intermarriage between the repatriates from Babylon and the local population, and conflict on both these issues between the priestly editors and government circles. To these issues there was a conflict about the uses of the doctrine of defilement. Ezra uses the doctrine to justify refusal of intermarriage, and claims it to be part of the law of Moses. In fact, this is the normal use of concepts of impurity the world round, but it is not condoned in the priestly teachings on impurity. The unique levitical doctrine would have been elaborated in response to these grave political problems. The priestly writings on defilement focus on protecting the tabernacle from various polluting conditions which every living being necessarily incurs. The human body is symbolically assimilated to the altar, and the sum of the laws of impurity provide a microcosm of God's creation. With this, biblical impurity is universalized in such a way as to prevent it being used as a social or political weapon.