As learning takes place, or is completed (i.e., all
available examples have been seen), the system
becomes available for reasoning. In this section, we
characterize PDL2's reasoning scheme, discuss how
PDL2 overcomes many aspects of brittleness, and
show how PDL2 handles the important commonsense
problem of inheritance. We follow the
characterization of the brittleness problem given in
[20]. Insightful (and more general) discussions of
commonsense, its implications, its nature and its
manifestations can be found in [1, 5, 12].