The notion of memory is central here, although in the world that Deckard and his quarry inhabit, it can't be relied upon. That's because replicants - essentially slaves with a built-in expiry date of four years - can have fake memories implanted by their creators, the Tyrell Corporation, to avoid detection.

Although the replicants Deckard is tracking know they are replicants, his love interest Rachael (Sean Young) believes she is human thanks to these false memories.

But memory underpins the film at a deeper level, particularly in the meditations of Roy Batty (Rutger Hauer), the leader of the replicants, who sets out to meet his maker in an attempt to extend his life span. Having found (and killed) his creator, Batty tussles with Deckard on the roof of a skyscraper, seemingly determined to kill him before ultimately saving his life.

In his final moments - and in 's most poetic lines of dialogue - Batty ponders a question that has plagued humanity since the dawn of consciousness: what happens to my memories when I die? "I've seen things you people wouldn't believe. Attack ships on fire off the shoulder of Orion. I watched C-beams glitter in the dark near the Tannhauser Gate. All those moments will be lost in time, like tears in rain."

Yet Batty, in a sense, discovers the answer to keeping his memory alive: by saving Deckard's life, he makes sure at least one person will remember him after his death. Indeed, it's arguable that by this act, Batty shows himself to be more human than Deckard, whose job is to routinely bring death.

The film adds another layer to this discourse with a clever final twist that has sparked one of the most hotly debated enigmas in cinematic history. If we can't trust Deckard's memories, we are left to wonder, just who can we trust?