Milgrom is aware of the need to provide some historical justification for his theory of the discontinuity between biblical and rabbinic notions of how impurity reaches the sanctuary. In his own view, the discontinuity is not large; yet, objectively, it is considerable, the Bible having the miasmic view of impurity impinging on the Tabernacle from all areas of the camp, while the rabbis think that no impurity can arrive at the Tabernacle unless some impure human individual brings it there.

Milgrom's theory is in some ways more congenial to the modern mind than that of the rabbis. For Milgrom provides an evolutionary scheme, by which we can see how demons evolved into ethics, while the rabbis provide a static scheme in which there is no development from the scriptural to the rabbinic system.

The demons, Milgrom argues, are not actually in Scripture, but they lie in its background, and occasional hints (such as the mention of Azazel) provide traces of a previous demonic theory of pollution which the priestly authors are endeavouring to overcome. Milgrom considers that the P document represents an enormous advance on ideas of demonic pollution, but has not quite emancipated itself from them. Previous Mesopotamian fears of temple-pollution derive from an active belief in demons, which were thought to threaten the Temple and even the god. Such evil forces, threatening to invade the temple from outside, had to be diverted by procedures of exorcism and cleansing. P has got rid of the demons, and denies that there is any threat to the omnipotent God.