I.2.6.
Systems of production
a) Systems of
production of the rules
One of the most popular types of the expert systems currently
is that based on the rules. The rules are popular for various reasons:
· Modularity: This makes easier to encapsulate knowledge
and to extend the expert system starting from an increasing development.
· Means of explanation: It is simple to build means of
explanation with rules because the antecedents of a rule specify exactly what
is necessary to activate it. By keeping the register of the rules which
discharged, a means of explanation can present the chain of reasoning which led
to a certain conclusion.
· Similarity with the human cognitive process: While
being based on the work of Newell and Simon, the rules appear as a natural
model in the way in which the human ones solve the problems. The Representation
IF... THEN rules facilitates the explanation of the specialists concerning the
structure of the knowledge which one tries to obtain from them. Other
advantages of the rules are described in Beam-Roth (85).
The rules are a type of production whose origins go back to
the Fifties. Had with the importance of the systems based on the rules, it is
worth the sorrow to examine the development of the concept of the rule. This
will give a better idea on why the systems based on the rules are so useful for
the expert systems.
b) Systems of
production of Post
The system of production was used initially in symbolic logic
by Post, which gave the origin to its name. It checked the significant one and
surprising result that unspecified mathematical system or logic could be
written like a certain type of system of the rules of production. This result
established the great capacity of the rules of production to represent larger
classes of knowledge instead of being limited to some types. Under the term
to rewrite rules, these one are also used in linguistics
like a manner of defining the grammar of a language. The languages of computer
are usually defined by using the form Backus-Naur (BNF) rules of production.
The basic idea of Post was that unspecified mathematical
system or logic represented only one whole of rules which specified how to
change a succession of symbols into another. I.e., being taken an action
pursuant of entry, the antecedent, a rule of production could produce a new
continuation, the consequence. This idea is also valid with programs and expert
systems in which the initial continuation of the symbols is consisted by the
input data and the continuation of exit is a transformation of what was
introduced.
Here is a simple case, let us suppose that the chain of entry
is "the patient has the fever", the chain of exit could be "take an aspirin".
Note that there is no significance which binds the two chains. I.e. the
handling of the chains is based on syntax and not on the semantics or the
comprehension of what fever, patient or aspirin mean. Human knows what these
chains in terms of the real world mean, but a system of Post production is only
one manner of transforming a chain into another. A rule of production for this
example could be:
Antecedent ? Consequence
Does the patient have the fever? Let him take an aspirin;
Where, the arrow indicates the transformation of a chain into
another. This rule can be interpreted in terms of the more familiar notation
IF... THEN in this manner:
IF the patient has the fever then let him take an aspirin
The rules of production can also have multiple antecedents.
For example:
The patient has the fever and
Is the fever with more than 39 ? let him see the doctor
Note that the special connector "and" does not form part of
the continuation; it states simply that the rule has several antecedents.
A system of production of Post consists of a group of rules of
production, as the following ones (the figures between brackets are there for
ends of analysis):
(1) Doesn't the car start ? let revise the battery
(2) Doesn't the car start ? let revise the fuel
(3) To revise the battery and if the battery is bad ? to
replace the battery
(4) To revise the fuel and if there is no fuel ? let fill the
fuel tank
expert system.
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