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The Protection of the Mezuza teaches that we are commanded to place the mezuza on the doorpost facing the public domain within the tefaḥ closest to the outside.
In discussing why this is the requirement, the Rabanan say that it is simply so that the mezuza will be reached immediately upon entering a house. Rabbi Ḥanina of Sura suggested that it is so that it will protect the entire house. In this context, the Gemara quotes the following in the name of Rabbi Ḥanina:
Come and see how the character of the Holy One, blessed be He, differs from that of men of flesh and blood. According to human standards, the king dwells within, and his servants keep guard on him from without; but with the Holy One, blessed be He, it is not so, for it is His servants that dwell within and He keeps guard over them from without; as it is said, The Lord is thy keeper; the Lord is thy shade upon thy right hand (Tehillim 121:5).
What is this protection?
We explore the mythic / magical side of this discussion.