Field Trip

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 ...
Kial was disgusted with the slow, cumbersome train. He disliked using this uncomfortable means of travel, but since he wanted to learn more about these strange creatures who were his ancestors, he had decided to try to become used to their ways.
He was lonely in this strange, backward age and when he unexpectedly saw another being like himself in the same coach, he hastened to make his presence known. He introduced himself and asked politely:
When are you from?
8000, the other replied. Name's Broyk, from VII Galaxy.
I'm from out XIX way myself, Kial said. Just a country boy. But 8000—that's only a period ahead of my own time. Maybe you could tell me ...
Ah, ah! the other admonished. Remember the First Law of Thek!
Oh, Center, Kial grumbled. I know: 'One may not divulge any scientific, technical or social information to anyone from his own past whom he may meet at an equidistant point in a Thek-travel.' I forgot.
Bad, Broyk 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.