Deckard and Rachael are both certain of their humanity. After a lengthy test, Deckard disocvers that Rachael is a replicant... but she doesn't know.
Deckard dreams of unicorns, and finds an origami unicorn at his doorstep, courtesy of Gaff. Is Deckard, then, a replicant?
Identity is something that is no longer certain, but instead becomes a construction that can be (like any structure) cracked or demolished. Huamnity becomes a prop, it's circular motion flowing from Deckard to Racael to Roy and back, always in question, always needing reassurance.