Field Trip by Gene Hunter description:
It is rumored that technology might eliminate many useless items from our regulated life of the future—including good, old-fashioned sex. However, let's kibitz for a moment …
oyk said. Then he added, almost jokingly: "You wouldn't want to be marooned in this dismal era, would you?"
Kial shuddered. "Of course not. But the Laws seem so ridiculous."
"Not a bit," Broyk said, warming up to the subject. "It's very simple, really. Same principle that doesn't allow anyone to Thek-travel into the future.
"Look. I'm from 8000. Say that I went into 12,000, where I memorized as much information as I could on some subject such as medicine. So I return to 8000, retaining all such knowledge in my mind that's been learned in four periods. Therefore, I'd have knowledge that wasn't dreamed of in my own time, but was discovered sometime during the next four periods. But then it couldn't be discovered, because I'd brought it back to 8000 and–well, I'm no Logician, but you see my point."
"Oh, it's reasonable, I suppose," Kial admitted. "I realize the Laws are really for our own go