The central problem in open domain-question answering from text is the problem of entailment. Given enough text, the answer is almost certain to be there, but is likely to be expressed differently than in the question-either in a paraphrase, or in a sentence that entails or implies the answer. We cannot afford to bridge this gap by open-ended theorem-proving search. Instead we need a semantics for natural language that directly supports common-sense inference, such as that arriving somewhere implies subsequently being there, and invading a country implies attacking it. We would like this semantics to be compatible with traditional logical operator semantics including quantification, negation and tense, so that not being there implies not having arrived, and not attacking implies not invading.
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