(9th-July-2020)
• At the top is entity. OWL calls the top of the hierarchy thing. Essentially, everything is an entity.
• Entities are either continuants or occurrents. A continuant is something existing at an instant in time, such as a person, a country, a smile, the smell of a flower, or an email. Continuants maintain their identity though time. An occurrent is something that has temporal parts such as a life, smiling, the opening of a flower, and sending an email. One way to think about the difference is to consider the entity's parts: a finger is part of a person, but is not part of a life; infancy is part of a life, but is not part of a person. Continuants participate in occurrents. Processes that last through time and events that occur at an instant in time are also both occurrents.
Querying Users and Other Knowledge Sources
• As discussed in Section 5.3.2, users are not experts in the domain of the knowledge base; however, they often know details about a particular case and so provide one source of knowledge. Users, typically, do not know what is relevant or what vocabulary to use and so they cannot be expected to tell the system what they know. One aspect of the problem of knowledge acquisition is how to most effectively extract knowledge from a user.
• The simplest form of a question is the yes-or-no question presented in Section 5.3.2. When variables and function symbols are involved, more sophisticated questions can be asked of the user.
• Example 13.17: Consider the knowledge base of Example 12.11, but without the rules for up or down. Suppose the user can observe the positions of the switches. To enable the user to be asked the position of switches, up(S) and down(S) can be askable. The following is a possible dialog for a top-down proof procedure for the query ?lit(L), where the user is asked the askable goals. User responses are in bold:
• Is up(s2) true? yes.
• Is up(s1) true? no.
• Is down(s2) true? no.
• Is up(s3) true? yes.
• Answer: L = l2.
• In this example, up and down are not explicitly related, and so the system asks both.
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