Originally Posted by Stripe
How do you know?
"Unknown" here can't mean something that can never be known. I'd call that "unknowable" or something. So, "unknown" can also be understood as being an answer to some problem. It is only "unknown" as long as we haven't solved the problem. Therefore, for some "unknowns" to be harder to turn into "knows" means that some problems are more difficult to solve than others.
For example, a in mathematical problem "x + 3 = 8", the x has an unknown value until we calculate it. It is easy to come up with a more difficult problem, having an "unknown" that is harder to turn into "known" than in the previous problem.