Well we might not know what electrostatic attraction is, but google "magnetism relativity" and you'll find that magnetism comes from relativistic effects of the motion of electrons. It's really nifty, and simple to understand if relativity is already a concept you're familiar with.
(That might be what you meant, but learning this a couple years ago blew my mind so I had to share.)
Yes, thats it! It turns out we (humanity) did know more, its just that I did not :) Thanks.
I think Feynman is explaining here that the only truly meaningful way for him to answer is to teach everything relevant to the question he knows that the other person doesn't; which can easily be a lifetime or several years worth of material.
The fields are presently among of our base concepts. Like the atom was, until the discovery of the electron and nucleus around ~1900.
What he does capture well is that you always have some kind of a base which you accept.