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3.4 Conditions for Abnormal Action Disqualications
Up to this point our discussion of the Qualication Problem revolved around
the questions both of how to assume away abnormal disqualications and
of how to withdraw this assumption in case some condition for the corre-
sponding abnormality is entailed. A related task is of equal importance for
reasoning about actions in environments which are unpredictable insofar as
success of actions cannot be guaranteed. Suppose an action turns out unex-
ecutable. This should come as a surprise whenever all regular preconditions
were satised and there was no reason to believe in an abnormality. Having
nonetheless encountered the disqualication, it is both natural and reasonable
to seek an explanation. An autonomous agent, for instance, whose current
goal relies on the successful performance of the action that failed should try
to gure out what went wrong in order to rectify it, if possible. This raises
the question of what qualies as an adequate explanation for an abnormality.
The natural thing to do if an abnormal disqualication has been observed
is to search the available domain knowledge for conditions which would entail
it. Among all potential causes those oer an explanation which are compatible
with the entire state of aairs. Suppose, for instance, our protagonist knows
that starting the car is impossible if some object clogs the tail pipe, if the
tank is empty, the battery is low, or if there is a general problem with the
engine itself. Suppose further that she has checked both tail pipe and tank
and has also conrmed the good status of the battery, say, by turning on the
radio. To the best of her knowledge, then, failing to start the car must be
caused by an engine problem.
On the formal side, this generation of explanations for observed abnor-
malities requires a specic policy of designing the state constraints that relate
action disqualications to their conditions. To see why, suppose the following
four constraints specify all that is known as to reasons for failing to start a
car.
)
tank-empty
disq
(
ignite
)
low-battery
9x:
in
(
x
)
disq
(
ignite
(3.2)
)
engine-problem
disq
(
ignite
)
disq
(
ignite
The conjunction of these formulas is logically equivalent to
9x:
in
(
x
)
_
tank-empty
_
low-battery
_
engine-problem
disq
(
ignite
)
(3.3)
This being a fluent formula that any acceptable state must satisfy, whenever
at least one of the four (abnormal) causes for a disqualication of
ignite
holds at some point, then
disq
(
ignite
) holds as well. The converse, how-
ever, is not necessarily true. That is to say,
disq
(
ignite
) may hold in a
state without any of the potential causes on the left hand side of the above
implication being true. Even worse, the four conditions supposedly belong
