New here. Trying to sort out the jargon and what not, but have immensely enjoyed reading through the posts. Stumbled upon this one and, I realize it's ten years old, but I have criticism: In the initial example, the example upon which the rest of the post was built, the Uncle was actually correct. We do NOT know how gravity works. We know how it acts upon the physical world, but as we've never discovered gravitational force carriers, we've zero knowledge of the mechanism through which it manifests.
Also, this certainly depends on how extreme you care to be, but you can certainly say with certainty whether or not science knows a thing. Example: no scientist on earth knows with any degree of scientific certainty whether or not God exists.