The which smell when the evil spirit had smelled, he fled into the utmost parts of Egypt, and the angel bound him.
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Both “desert” and “Egypt” refer to the hearts of unbelievers, which are deserts, that is, forsaken by God of whose indwelling they are unworthy; and according to the meaning of the name “Egypt,” these same hearts are plunged into shadow by the darkness of their own unbelief. And not without reason is the one who is deserted by the grace of divine light filled by the prince of darkness. The reason why the angel got hold of the demon that wanted to kill Tobias and bound him in the desert of upper Egypt is that the devil, while restrained from snatching away the faithful who are members of their Redeemer, is allowed by this Lord and redeemer of ours to have dominion over unbelievers only. And even in their case the Lord holds him bound because even the wicked, the very ones of whom he is master, he is not allowed to harm as much as he longs to in his insatiable rage. - "On Tobit 8.3"