I think some mathematical concepts are difficult to learn
   because they're taught poorly. Archaic language. It's not a
   natural language. I see mathematical constructs as a subset of
   natural language constructs. There's a lot to ignore that are
   are normal in natural language. Within the brain, afaik,
   mathematics beyond numerousity (I think that's the term) is
   linguistic. WE instantly assess - mechanically - 3 objects at
   once, maybe 4. But beyond that, it's a language with its own
   grammar.