How Much Do We Really Know?
It's hard to answer that question without getting into semantics of what it means to "know" or to "have knowledge". I'll bypass all these sometimes sophisticated and often cumbersome arguments to say that whatever knowledge is, is gradational.
That is, we know some things "more" or "less", depending on our information on the subject matter, our position in life, our experiences and all these other factors that are extremely difficult to enumerate, because there are so many.
Having said this, I think there is good historical evidence and indeed some simple questions one can ask to find out how
much we know. I'll keep coming to physics, not because it is the most important subject - I don't think there is such a thing, - but because our knowledge of it is the best tested knowledge we have. All other knowledge we have in other areas of life pale in comparison to the quality of evidence we have in physics.
So ask a simple question: "what is gravity?", "what is a particle?", "what is magnetism?". The answers given are
only the
effects we can perceive of the phenomena. As to what these things are, we don't know.
Now go up in complexity to chemistry, biology all the way up to psychology. We multiply particles by billions. Minds enter the fray as do complex emotions. This complexity, if you stop and think about it, is truly mind-boggling. As the saying goes, if our knowledge is limited - as it is - our ignorance is infinite.