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Error Correction: The Specialization Theory 213

To develop this view of declarative knowledge, we need a formal theory,
a logic of constraints to rival the logic of propositions. This requires assump-
17
tions about how constraints are represented in memory and how they are pro-
cessed. I propose that a constraint can be represented as an ordered pair
< CC > ,
,
r
s
in which C is a relevance criterion, that is, a specification of the conditions
r
under which the constraint is relevant, and C is a satisfaction criterion, that is,
s
the conditions that have to be met for the constraint to be satisfied. Each part
of the constraint is a conjunction of expressions that refer to situation features.
If the features specified in C hold in a particular situation, then the features
r
specified in C ought to hold in that situation as well, or else the constraint is
s
violated. If C does not hold, then it is irrelevant whether C holds.
r
s
Many familiar instances of declarative knowledge can be reinterpreted as
collections of constraints. To continue with the traffic example, my knowledge
that ABC Avenue is a one-way street constrains how I drive on that street. If
I drive in the approved direction, my action satisfies the constraint; if I drive
in the opposite direction, my action violates the constraint. Expressed in the
formal representation, the knowledge that ABC Avenue is one-way in the east-
erly direction becomes this: If you are driving on ABC Avenue, you ought to be
driving in the easterly direction. In this example, driving on ABC Avenue is the
relevance criterion and driving in the easterly direction is the satisfaction crite-
rion, so this constraint can be formalized as
(Driving x) & (On x ABC-Avenue) ** (Direction x East)
The “**” symbol stands for the connective “ought to.” If I am walking or if I
am not on ABC Avenue, the direction in which I am traveling is of no impor-
tance from the point of view of this particular traffic regulation; that is, the
constraint is irrelevant. But if I am driving and, furthermore, driving on ABC
Avenue, that is, if the conjunction
(Driving me) & (On me ABC-Avenue)

is the case, then I had better be driving east rather than west, meaning the
expression

(Direction me East)
ought to hold as well. If I drive westward, I violate the constraint, which implies
that I made a wrong turn.