Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

∴ q is true.
2) Defeasible derivation: It has already been mentioned that defeasible
rules employ --> type of implication.
Given that, the knowledge base includes p -->q, where p is a conjunction
of literals p1, p2, ---,pn. Now q is defeasibly derivable if
a) q is absolutely derivable by another absolute rule rq, where r is
either available in the knowledge base or absolutely derivable.
or b) p1, p2, ---, pn all are defeasibly derivable, if p q is not defeated, i.e.,
p /-->¬ q is guaranteed.
or c) p --->q is not defeated, when p1, p2, -- ,pn all are defeasibly derivable
literals.
3) Defeating: An absolute rule p q can be defeated by an absolute
contradictory rule or fact. A defeasible rule p--> q can also be defeated by
a defeasible rule r---> ¬q or r -/--> q, if p is not ‘more restrictive’ than r.
The definition of ‘more restrictiveness’ will be presented under c(ii).
A rule pq is defeated by (a) and (b) below, while a rule p---> q is
defeated by (a), (b) and (c) below.
(a) ¬q is available in the knowledge base.
(b) r¬q is present in the knowledge base, where r is a conjunction of
defeasibly derivable literals [1].
(c) r is a conjunction of defeasibly derivable literals rj , so that
i) r --> ¬ q or r --/--> q is in the knowledge base.
ii) One or more rj is not absolutely derivable from p1, p2, .., pn and
the absolutely derivable rules in the knowledge base.
The principles stated above are illustrated with the following example.
Example 8. 4: Consider the following knowledge base:
Rule1: Person(X) ---> Mortal(X)
Rule2: Poet(X) -/--> Mortal(X)
Rule3: Person (tagore).
Rule4: Poet (tagore).