I think that Natural Language Understanding is absolutely linked to
the Symbol Grounding problem.  The Symbol Grounding Problem states
that a the meanings of symbols are "grounded" and get their meaning
from the thoughts going through one's head.  This explains many of the
problems that Natural Language Understanding has with interpreting
sentences.  Going the other way, searching for formal definitions for
each word that makes sense given the formal definitions of the other
words leads to two problems.  One is infinite regress: definitions
built off of definitions built off of definitions.  The second is the
problem of who is deciding which combination of rules make sense?  If
that could be achieved, the string of definitions wouldn't be needed.
To truly understand a language, one must have an independent
understanding of the word expressed.