Tweedle Dee and Tweedle Dum

Tweedle dee and Tweedle dum,
were sitting here upon my thumb,
when one squeak exclaim he,
that we were not,
but we were we.
Said Tweedle Dum to Dee,
you foolish silly,
we are not we,
but we are us,
and Dum and Dee caused quite a fuss,
racket and clacket of this and thus
about who we were
and what we are
this and that and even so far
to say that us is not we
and we are not we
and so they argued endlessly
of the possiblities
of something useless as
us.. or we...
or were or me..
What could it be..?