We may say that in language ([9|11]) we have various parts of speech. For the functions of “slab” and “cube” are more alike than the functions of “slab” and “d”. But
the way
how
we c[al|la]ssify the words together as various parts of speech will depend on the purpose of the classification, and on our inclination.
      Think of the different points of view
according to
from
which one mi[h|g]ht classify tool[d|s] as different kinds of tools. Or chess pieces as different kinds of pieces.