Who says that the devil lacks a moral sense?
Or is he just erudite? Let us see . . . . In Paradise Lost 4.55-57, Satan recognizes:
. . . that a grateful mind
By owing owes not, but still pays, at once
Indebted and discharged; what burden then?
As Alastair Fowler points out (Paradise Lost (1998), 218, n. ll. 55-7), Milton borrowed from Cicero, Pro Plancio xxviii 68 to put these words on the devil's otherwise forked tongue:
In a moral debt, when a man pays, he keeps, and when he keeps, he pays by the very act of keeping.
At a 2003 conference on Johannine studies held in St. Andrews, Scotland, I prefaced my paper on "Gift-Giving in John's Gospel" with Milton's Satanic quote but left the devil unacknowledged to see if anyone would recognize the source.
No one did.
I now acknowledge my debt to Old Scratch himself, who in turn should acknowledge his debt to Milton, who should further fulfill the obligation of acknowledging his debt to Cicero, who . . . well, where did Cicero get this idea?