Open World Assumption
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The open world assumption or OWA assumes that its knowledge of the world is incomplete. If something cannot be proved to be true, then it doesn't automatically become false. In the OWA, what is not stated is considered unknown, rather than wrong. It is popular with databases, especially for querying the database. The open world assumption is considered implicit in RDF and OWL, as every tuple not explicitly contained in the semantic web or ontology is implicitly assumed to represent a fact that is unknown [rather than false].
Example
Statement: "Mary" "is a citizen of" "France"
Question: Is Mary a citizen of Canada?
"Closed world" (for example SQL or XML) answer: No. "Open world" answer: unknown (Mary could have dual citizenship).
Under OWA, failure to derive a fact does not imply the opposite. For example, assume we only know that Mary is a citizen of France. From this information we can neither conclude that Mary is not a citizen of Canada, nor that she is. Therefore, we admit the fact that our knowledge of the world is incomplete. The open world assumption is closely related to the monotonic nature of first-order logic: adding new information never falsifies a previous conclusion. Namely, if we subsequently learn that Mary is also a citizen of Canada, this does not change any earlier positive or negative conclusions.
