We can [in theory] never know everything about human thinking.
I wanted to suggest with Copenhagen that there is some kind of parallel between the indeterminacy of human thinking, and the indeterminacy that Heisenberg introduced into physics with his famous Uncertainty Principle.
Though I'm not trying to say they're exactly parallel.
The uncertainty principal says that there is no way, however much we improve our instruments, that we can ever know everything about the behavior of a physical object.
And I think it's also true about human thinking.