Rules for Knowledge
Representation
One way
to represent knowledge is by using rules that express what must happen or what
does happen when certain conditions are met.
Rules are
usually expressed in the form of IF . . . THEN . . . statements, such as: IF A
THEN B This can be considered to have a similar logical meaning as the
following: A→B
A is
called the antecedent and B is the consequent in this statement.
In
expressing rules, the consequent usually takes the form of an action or a
conclusion.
In other
words, the purpose of a rule is usually to tell a system (such as an expert
system) what to do in certain circumstances, or what conclusions to draw from a
set of inputs about the current situation.
In
general, a rule can have more than one antecedent, usually combined either by
AND or by OR (logically the same as the operators ∧ and ∨).
Similarly,
a rule may have more than one consequent, which usually suggests that there are
multiple actions to be taken.
In
general, the antecedent of a rule compares an object with a possible value,
using an operator.
For
example, suitable antecedents in a rule might be
IF x >
3
IF name
is “Bob”
IF
weather is cold
Here, the
objects being considered are x, name, and weather; the operators are “>” and
“is”, and the values are 3, “Bob,” and cold.
Note that
an object is not necessarily an object in the real-world sense—the weather is
not a real world object, but rather a state or condition of the world.
An object
in this sense is simply a variable that represents some physical object or
state in the real world.
An
example of a rule might be
IF name
is “Bob”
AND
weather is cold
THEN tell
Bob ‘Wear a coat’
This is
an example of a recommendation rule, which takes a set of inputsand gives
advice as a result.
The
conclusion of the rule is actually an action, and the action takes the form of
a recommendation to Bob that he should wear a coat.
In some
cases, the rules provide more definite actions such as “move left” or “close
door,” in which case the rules are being used to represent directives.
Rules can
also be used to represent relations such as:
IF
temperature is below 0
THEN
weather is cold
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