i would ask the same thing to you: how do you "know" that we cannot not "know" anything if not by deduction?
oh, so you aren't REALLY sure that we can't know anything then? it's not an immutable 'fact' of reality then? that would make it possible to know something... COOL!
But we still don't know if "knowledge by deduction" is really knowing. Furthermore, we don't know what knowing is. That would be why I said if you could answer my question that I would concede that it is not a "fact". Yes, provided we knew what "knowing" was and we as humans knew that "knowing" was within our capabilities, it would seem to be possible to know something. unfortunately we still don't know what knowing is. I now know (or believe to have knowledge in this case) what its like to be a computer caught in an infinite loop.
This is getting close to analysis to paralysis. Somewhere, at least in trading, you have to pull the trigger.