Like many great films (The Godfather (1972), Jaws (1975), One Flew Over the Cuckoo’s Nest (1985)), Blade Runner (1982) is actually based on a novel. Written by sci-fi legend Philip K. Dick, Do Androids Dream of Electric Sheep? follows a bounty hunter named Rick Deckard who’s hired to hunt down a group of escaped androids. Along the way, he meets a beautiful android named Rachael, goes up against two villains named Pris and Roy Baty (one less “t”), and uses the Voight-Kampff test to determine who’s human and who’s not.

And that’s kind of where the similarities end.

The most notable difference between the novel and the film is probably the absence of the term “blade runner.” The movie title was actually inspired by the name of a 1979 William S. Burroughs novella, however in the book, Deckard is simply referred to as a bounty hunter. The word “replicant” never shows up either. That catchy name was coined by screenwriter David Peoples, and in the book, Roy, Pris, and Rachael are referred to as “andies.” Similarly, the movie takes place in a crowded Los Angeles whereas the novel is set in a nearly abandoned San Francisco.

So why is The City by the Bay deserted? Well, Do Androids Dream of Electric Sheep? is set after World War Terminus (WWT), a deadly conflict that’s destroyed the Earth with its radioactive fallout. Most of the planet’s animals have gone extinct, and a large amount of the human population is suffering from physical and mental defects. These unfortunates are nicknamed “chicken heads,” and the novel introduces us to one of these handicapped humans, an ambulance driver named J.R. Isidore. Fans of the film can probably guess that Isidore is the inspiration for J.R. Sebastian. Of course, while Isidore is mentally challenged, Sebastian is an inventor-genius suffering from Methuselah Syndrome.

Thanks to the radiation, most of the healthy humans have immigrated to off-world colonies where they’re paired with robotic servants (andies). Of course, there are a few “normal” people who’ve elected to stay on Earth, such as Deckard. The remaining humans are obsessed with animals, and since most creatures were wiped out thanks to WWT, live pets have become a major status symbol. Those who can’t afford real animals often buy robotic pets (like Deckard’s titular electric sheep), and that’s why Deckard is hunting the andies. He wants to earn enough cash to buy a living, breathing pet.

The film also does away with mood organs, devices that alter the way people feel, and skips over an entire subplot involving a religion known as Mercerism. Believers around the galaxy use a special device called an empathy box to form a collective consciousness and fuse with a saint named Wilbur Mercer. By experiencing his trials and tribulations, followers are able to share experiences and empathize with one another.

Really, you could summarize Dick’s novel with that one single word: “empathy.” It’s the central theme of the novel, especially where the andies are concerned. Dick describes empathy as the defining trait of humanity; it’s what makes us different from every other creature, including the androids. This is where the novel and the film majorly diverge. Whereas Ridley Scott portrays the replicants as beings who can feel emotions like love, fear, and guilt, Dick paints a very different picture.

In one scene in the novel, a group of andies mercilessly torture a spider, and when J.R. Isidore protests, the robots are completely baffled by his distress. In the film, Rachael (Mary Sean Young) is a tender-hearted woman who falls in love with Deckard (Harrison Ford). In the novel, she’s a manipulative villain who kills Deckard’s pet out of spite. According to Dick, the andies can never empathize with others, and that’s the key difference between man and machine. In his own words, the replicants are “deplorable because they are heartless, they are completely self-centered, they don’t care what happens to other creatures, and to me this is essentially a less-than-human entity for that reason.”

True, in the movie, Roy Batty (Rutger Hauer) and his friends are violent criminals. They murder quite a few people who get in their way, but they never kill out of some sort of psychopathic detachment, because they can’t empathize with their victims. They murder because they’re afraid of dying, because they’re plagued by the fear of the unknown. And when Deckard dangles from the roof of Sebastian’s apartment, Roy reaches out and hauls his enemy onto the roof. “I don’t know why he saved my life,” the blade runner says in a 1982 voice over. “Maybe in those last moments he loved life more than he ever had before. Not just his life…anybody’s life…my life.”

In short, the Batty of Blade Runner is a flawed hero who empathized with Deckard in his last moments. The Baty of Dick’s novel would’ve simply walked away.