Let me start by pointing to to https://news.ycombinator.com/item?id=7715277.

How do you know something exists?

Because you can see it.

So you don't know that something exists, you only know that your sense impressions have been triggered. How do you know its nature?

Because of what you see.

So you don't know anything about its nature, only about its relationship with your sensory apparatus. (Hey, I may have just invented category theory!)

How can something exist...?

Don't look at me, man. It's all probability waves as far as I can tell.

Note: I don't really, actually, myself, buy into this form of epistemology and metaphysics, so I may have the arguments badly. On the other hand, those are major topics that have a lot of diverse implications. They cannot simply be dismissed by assertion.