(16th-August-2020)
• A database is often complete in the sense that anything not stated is false.
• Example 5.24: You may want the user to specify which switches are up and which circuit breakers are broken so that the system can conclude that any switch not mentioned as up is down and any circuit breaker not specified as broken is okay. Thus, down is the default value of switches, and okay is the default value for circuit breakers. It is easier for users to communicate using defaults than it is to specify the seemingly redundant information about which switches are down and which circuit breakers are okay. To reason with such defaults, an agent must assume it has complete knowledge; a switch's position is not mentioned because it is down, not because the agent does not know whether it is up or down.
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• The definite clause logic is monotonic in the sense that anything that could be concluded before a clause is added can still be concluded after it is added; adding knowledge does not reduce the set of propositions that can be derived.
• A logic is non-monotonic if some conclusions can be invalidated by adding more knowledge. The logic of definite clauses with negation as failure is non-monotonic. Non-monotonic reasoning is useful for representing defaults. A default is a rule that can be used unless it overridden by an exception.
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