Intro
Derrida got into a famous face-off with Lacan over this detective story. How's that? Well, put your eyeballs to this collection of essays, where the answer to that critical riddle is hidden in plain sight.
Quote
Why—it did not seem altogether right to leave the interior blank—that would have been insulting.
Analysis
These words are spoken by Dupin, Poe's proto-Sherlock Holmes. And we hope it won't be giving too much away to say that the detective here plays a little Derridean trick: inscribing something in a letter that's not the letter the addressee is expecting to find.
Remember Derrida's whole little ditty about how literature should always be messing with readers' expectations, and readers should always be preparing themselves for fun surprises?
While Lacan thought that Poe's story proved "the letter always arrives at its destination," Derrida thinks that "The Purloined Letter" is about a series of unanticipated mistakes. (This might remind you of The Post Card.)