Oh, gracious, this one will be never-ending.
The gist of the part with which you are having difficulty is this: All that we presently know of language, and which we believe to be true of it, is . . . . .
From there, you will have to interpret scores of replies.
Good luck!! (And I mean it!)
(Not a Newbie, but I like the various faces for this post!)
Student or Learner