Piper in the Woods by Philip K. Dick
“Well, Corporal Westerburg," Doctor Henry Harris said gently, "just why do you think you're a plant?"
As he spoke, Harris glanced down again at the card on his desk. It was from the Base Commander himself, made out in Cox's heavy scrawl: Doc, this is the lad I told you about. Talk to him and try to find out how he got this delusion. He's from the new Garrison, the new check-station on Asteroid Y-3, and we don't want anything to go wrong there. Especially a silly damn thing like this!
Harris pushed the card aside and stared back up at the youth across the desk from him. The young man seemed ill at ease and appeared to be avoiding answering the question Harris had put to him. Harris frowned. Westerburg was a good-looking chap, actually handsome in his Patrol uniform, a shock of blond hair over one eye. He was tall, almost six feet, a fine healthy lad, just two years out of Training, according to the card. Born in Detroit. Had measles when he was nine. Interested in jet engines, tennis, and girls. Twenty-six years old.
"Well, Corporal Westerburg," Doctor Harris said again. "Why do you think you're a plant?"
The Corporal looked up shyly. He cleared his throat. "Sir, I am a plant, I don't just think so. I've been a plant for several days, now."
"I see." The Doctor nodded. "You mean that you weren't always a plant?"
"No, sir. I just became a plant recently."
"And what were you before you became a plant?"
"Well, sir, I was just like the rest of you."