This is from John Lennon I believe. The structure is confusing, and certainly "cryptic". Lennon was a utopian dreamer, which makes his song lyrics popular, but also leaves him open to justifiable criticism as to his actual knowledge. Rather than try to explain why the statements are either contradictory or false, it is simpler just to tell you that the are such that they sound pleasant and even "authoritative" while being entirely illogical. I haven't the time at the moment to see if these are from Lennon's song "Imagine"; but suffice it to say, that trying to "understand" statements like these will probably lead to much confusion, without some knowledge of Epistemology. .

1) There's nothing you can do that can't be done [just as well by someone else] 2) [And there's] Nothing you can sing that can't be sung [just as well by someone else] 3) [And there's] There's nothing you can make that can't be made [just as well by someone else] 4) [And there's] No one you can save that can't be saved [just as well by someone else] 5) There's nothing you can know that isn't [already] known So that all sort of makes sense. BY THE TIME WE GET TO THE NEXT ONE, 6) [And there's] Nothing you can see that isn't shown he needs to fit the rhythm, rhyme and pattern he's already set. Well, he got stuck. He couldn't quite do it. He got as close as he could. It would have made more sense to say Nothing you can see that can't be seen [by someone else] but "seen" doesn't rhyme with "known." We can "translate" it: "There's nothing YOU can see that is invisible to everyone else."

It's Lennon, the song is "All You Need Is Love," and you shouldn't expect song lyrics to make perfect sense. The first line, "There's nothing you can know that isn't known" makes sense to me. The second line doesn't quite make sense, he's just needs a rhyme and a parallel structure. And of course he's already said five different variations of this, "nothing you can do that can't be done, nothing you can sing that can't be sung" and is running out of perfect lines. "There's nothing you can know that isn't known" means "there's nothing YOU can find out or discover that hasn't already known by something else." It seems pessimistic. It is very similar in tone to a famous line from the book of Ecclesiastes in the Bible: "What has been will be again, what has been done will be done again; there is nothing new under the sun." In the context of the song, IN MY OPINION, Don't be too ambitious, don't too strive for achievement, there's nothing you can add to human achievement. "Love is all you need," concentrate on loving others and don't work too hard on doing, singing, making, saving, knowing, "showing."

what you know is known what you see is seen

Would it be clearer by adding "already"? ...that isn't already known/shown. Basically, we have a relative clause which describes a longer subject. "There's [nothing you can know] [that isn't known]." The subject is "nothing you can know"... well, it's actually a subject (nothing) with its own relative clause ((that) you can know).