For instance, Hank says the future cannot be known. But math and physical laws work against that view; for instance, as soon as I press "post" I shall add 2 plus 2 in decimal and I know the answer will be 4 in decimal.
Further, I perceive all that there is to include extra dimensions, one of which is time, and therefore time is a plane and not a line and thus, not only is the future, past and present known in that dimension but it may also be knowable in 4D.
Likewise, by the grace of God, I know Him and perceive many things which are not knowable to one who does not have the eyes to see or the ears to hear. But others who share the same Holy Spirit are of the same mind; we perceive spiritual matters as One being.
Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual. But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know [them], because they are spiritually discerned.
But he that is spiritual judgeth all things, yet he himself is judged of no man. For who hath known the mind of the Lord, that he may instruct him? But we have the mind of Christ. - I Corinthians 2:11-16
So what would be the point of trying to define "what cannot be known"?
Yes, we can know the future contingently, on the basis of, if this then thus'n'so, but, as soon as you press post you may drop dead (heaven forbid) or the power on your calclator may go out and you will not do what you intend. You just don't know.
...But we have the mind of Christ ...
Yes, but which. The mind of Christ as the risen ascended Redeemer, or the mind of Christ within the limits of human capacity, just as His was while in this world, for though He was given the Spirit without measure (John 3:34) yet He could not tell if there were figs on a tree without looking (Mark 11:13). So maybe one of the things we cannot know is whether or not there is fruit on a tree, at least without looking. (It is similar to the cat in the cupboard, if you followed that earlier).
So what would be the point of trying to define "what cannot be known"?
Possibly none beyond an interesting exercise. Surely there would be some things we all would agree cannot be known, and those we disagreed on might provide insights into why we disagree, and what the differences in our understanding of knowledge are really based on.
You've already provided some interesting insights.
Hank