“In 1951, when I sold my first story, I had no idea that such fundamental issues could be pursued in the science fiction field. I began to pursue them unconsciously. My first story had to do with a dog who imagined that the garbagemen who came every Friday morning were stealing valuable food which the family had carefully stored away in a safe metal container. Every day, members of the family carried out paper sacks of nice ripe food, stuffed them into the metal container, shut the lid tightly—and when the container was full, these dreadful-looking creatures came and stole everything but the can.
“Finally, in the story, the dog begins to imagine that someday the garbagemen will eat the people in the house, as well as stealing their food. Of course, the dog is wrong about this. We all know that garbagemen do not eat people. But the dog’s extrapolation was in a sense logical—given the facts at his disposal. The story was about a real dog, and I used to watch him and try to get inside his head and imagine how he saw the world. Certainly, I decided, that dog sees the world quite differently than I do, or any humans do. And then I began to think, Maybe each human being lives in a unique world, a private world, a world different from those inhabited and experienced by all other humans. And that led me wonder, If reality differs from person to person, can we speak of reality singular, or shouldn’t we really be talking about plural realities? And if there are plural realities, are some more true (more real) than others? What about the world of a schizophrenic? Maybe, it’s as real as our world. Maybe we cannot say that we are in touch with reality and he is not, but should instead say, His reality is so different from ours that he can’t explain his to us, and we can’t explain ours to him. The problem, then, is that if subjective worlds are experienced too differently, there occurs a breakdown of communication… and there is the real illness.
“I once wrote a story about a man who was injured and taken to a hospital. When they began surgery on him, they discovered that he was an android, not a human, but that he did not know it. They had to break the news to him. Almost at once, Mr. Garson Poole discovered that his reality consisted of punched tape passing from reel to reel in his chest. Fascinated, he began to fill in some of the punched holes and add new ones. Immediately, his world changed. A flock of ducks flew through the room when he punched one new hole in the tape. Finally he cut the tape entirely, whereupon the world disappeared. However, it also disappeared for the other characters in the story… which makes no sense, if you think about it. Unless the other characters were figments of his punched-tape fantasy. Which I guess is what they were.”
How to Build a Universe That Doesn’t Fall Apart Two Days Later, by Philip K. Dick, 1978. Deoxy.org