What do you really know?

If I haven't done it, seen it happen, tasted, smelled, heard, or felt it can I say I know it? Everything other than those I've been told, read, or rely on someone else for that knowledge, or come to my own conclusions taking those things into consideration.
Therein lies the conundrum. I am of the opinion that one does not need to experience something to know whether it is desirable, repugnant, foolhardy, or just to be avoided. For instance I would not want to be shot, stabbed, car wrecked, shipwrecked, or otherwise put through painful scenarios even if I've never experienced them. It is possible to learn from others who have been where you have not.
It is also possible to believe things exist that you've never seen or that you are likely never to see. I have to believe that UFOs are real only because of the number of people who have reported seeing them even without the photographic evidence of such things.
Other things are harder to believe or maybe said better to understand or know. I understand the theory of relativity and its implications vaguely but I can't do the calculations. Better yet, how does a quarterback complete a pass to a receiver when both are in motion and everything is literally up in the air? Same thing for a basketball player making shots from different places on the court; how does one gauge the force needed to complete the basket? These are things that have to be done to be known; is there a formula?
The bottom line is that I know what I know but you can't be sure about that and I must doubt you as well. Belief doesn't make anything so. Much knowledge and many things are unbelievable. Many beliefs are not valid nor true knowledge.
When in an adversarial situation it is important to know what the adversary believes so that you may have a better chance to thwart him than to assert your own beliefs because they matter not to him unless he is a truly worthy opponent.

Mahabuhay