1. At least one computer-readable storage medium comprising computer-executable instructions stored thereon that, when executed by a computer system, cause the computer system to: receive at least one document for processing; extract at least one expression from the document, the expression containing a scope relationship occurring between at least two terms contained in the expression; calculate a valence of the expression based on the scope relationship between terms as defined in the expression; enter one or more facts in a semantic index representing the valence calculated for the expression, the semantic index comprising an inverted index mapping facts representing valence calculated for expressions in a plurality of documents to the document in which each expression occurs; receive a natural language query that references sentiment expressed by a speaker or author; translate the natural language query into a representation to be compared with the facts in the semantic index; and retrieve documents from semantic index relevant to the natural language query.