Impostor: The Replicant Before Blade Runner

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This isn't exactly a story from the cyberpunk genre, but it has a few traits that, to me, place it in a kind of "proto-cyberpunk" category. Impostor, by Philip K. Dick — found through the Luminist Project — was published in Astounding magazine in June 1953, and depicts a world at war with an alien civilization.
That's typical science fiction, action-movie territory, not necessarily cyberpunk. But an important question is raised even before the story begins, one that changes everything that follows:
If it were known that The Enemy could manufacture a letter-perfect duplicate of a man – how could you prove you were yourself?
A replicant, like in Blade Runner (1982) and Do Androids Dream of Electric Sheep? (1968). Except this one came decades earlier.
That's what makes the story feel "proto-cyberpunk" rather than simply science fiction. The term refers to works that predate the formal emergence of the genre, which became established in the early 1980s mainly through authors like William Gibson and his 1984 novel Neuromancer. 
These earlier works already carried thematic and aesthetic elements that would later define cyberpunk, even though the label didn't exist yet. Things like invasive technology, powerful corporations dominating society, hackers, artificial intelligence, urban dystopias, the fusion of human and machine, and a deep distrust of technological capitalism. 
Impostor doesn't tick every one of those boxes, but its central question, about identity, duplication, and the impossibility of proving you are who you say you are, plants a seed that cyberpunk would later grow into something much larger.
Before cyberpunk had a name, Philip K. Dick was already asking: how do you prove you're human?
From here on, spoilers for the story, which is quite short – just over 10 pages.
Olham is a high-ranking officer responsible for a project that could finally give Earth an edge in the war against the aliens from Alpha Centauri, the Outspacers. He is suspected of having been replaced by a replicant (I'll use that term, even though it never appears in the story) without being aware of it himself. The replicant's programming gives it the real Olham's memories, and the only way to tell the real one from the fake is to "take him apart," since it supposedly carries a bomb inside, set to go off at the research lab, since the Outspacers found out about the project and its potential, and want to destroy it.
Olham, for his part, is convinced none of this applies to him. He knows he isn't a robot. He remembers details of his life, he was living it normally until shortly before being arrested. The problem is that knowing isn't the same as proving, and proving requires evidence. The perfect proof, he figures, would be the robot's body, which he assumes "died" in the crash of the ship, since it never actually reached its target.
Except it isn't quite like that. He does find the ship, and a dead body. But what's there isn't the robot, the replicant, it's the real Olham. And, of course, the bomb goes off, taking out everyone around, including the fake Olham, who realizes, in his final moment, that his certainty of being human was nothing more than a machine's trick.
It's that twist that makes the story feel like more than just a war tale. As I said at the start, this isn't a cyberpunk story, but rather something that carries a few of the genre's traits, even if only in an "embryonic" form. Specifically, it's a story about identity paranoia and the replacement of humans by technology.
That paranoia is the part that lingers. When technology gets too close to the human mind, the line between real and fake, biological and machine, becomes blurred, and it raises the question of how far modifications to the human body can go before crossing into the non-human. In Ghost in the Shell this theme comes up often, even though that small human spark, consciousness, is enough to deny the machine's artificiality.
It's a question that doesn't feel so far off anymore. 
A few days ago I saw news that a chatbot passed the Turing Test: when given a specific persona prompt instructing the model to embrace human fallibility, tone, and humor, GPT-4.5 reached a 73% human-deception rate, while without those explicit instructions, its success rate dropped to 36%. In other words, it's a matter of being given the right prompt, because the capability already exists.
Before cyberpunk had a name, Philip K. Dick was already asking: how do you prove you're human?
The Turing test is a method proposed by mathematician Alan Turing in 1950 to evaluate whether a machine can exhibit intelligent behavior indistinguishable from that of a human. The basic idea is simple. A human judge holds a text conversation with two hidden participants, one a person and the other a machine, without knowing which is which. If the judge cannot reliably tell the machine apart from the human based on the conversation alone, the machine is said to pass the test.