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. Semantic Web languages such as RDF(S) and OWL implicitly make the open world assumption. The absence of a particular statement within the web means, in principle, that the statement has not been made explicitly yet, irrespectively of whether it would be true or not, and irrespectively of whether we believe (or would believe) that it is (or would be) true or not. In essence, from the absence of a statement alone a deductive reasoner cannot (and must not) infer that the statement is 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.
