they will prove the truth of my tale
Again, the creature seems to echo Victor Frankenstein (though, since Victor's narrative to Walton postdates the Creature's by several years it is actually he who is engaged in echoing). Wherever in the narrative nesting we look to find a foundation for its truth, we discover both Victor and the Creature seeking to establish the evidence that will verify their accounts and using almost identical language to do so. One may thus compare this utterance with those of Victor to Walton at the beginning (I:L4:30 and note) and end (III:Walton:2) of his narrative. That the "evidence" comes from the centrally embedded narrative, the story of Safie, is thus taken to lend credibility to all narrative strands that subsume it. But, of course, these putative copies of letters depend entirely on the Creature's word for their authenticity: so there is actually no documentary foundation whatsoever for the "truth" of any of these fictions. Still, it is indicative that Victor Frankenstein produces the letters to convince Walton of his veracity (III:Walton:2).