2.2. Uncertainty of Precepts
To properly reflect the uncertain nature of
precepts, the system must have mechanisms to deal
with both correct and incorrect guesses. As a policy, a
precept is always assumed to be correct until enough
evidence is available (from experience with specific
examples) to the contrary. The question of what
constitutes enough evidence is naturally the key to
properly handling truly incorrect guesses. The danger
is that information might be lost if a precept is
mistakenly deleted. In [7], enough evidence is defined
as the existence of examples (or other precepts)
contradicting the targeted precept on more than half of
the input space it covers. Though reasonable, this
scheme often results in loss of information and is
inadequate (due in part to the representation language
used) for commonsense reasoning (particularly
inheritance). Since mechanisms exist to keep track of
exceptions and exceptions, because they are more
specific, have priority over more general rules, PDL2
does not need to remove precepts