There may be "nothing so practical as a good theory," but coming up with an
effective theory is often difficult. By definition, a theory, taxonomy, or model is
an abstraction of reality and therefore must be incomplete. However, a good theory
should at least be understandable, produce similar conclusions for all who
use it, and help to solve specific practical problems. This section reviews a range
of descriptive and explanatory theories, in preparation for the discussion of the
object-action interface model in the next section.