1. A method for producing a phrase thesaurus, comprising the steps of: providing access to a thesaurus building computer coupled with a machine-readable phrase thesaurus database; identifying, by thesaurus building computer, a plurality of valid phrases that occur within a text corpus; determining, by thesaurus building computer, the degree of similarity between the valid phrases, wherein one or more contexts are created for each valid phrase, wherein each context for a valid phrase comprises a word or phrase that appears adjacent to the valid phrase in the text corpus, and wherein the degree of overlap of the contexts of each valid phrase with the contexts of each other valid phrase is determined; and grouping, in the phrase thesaurus database, the valid phrases into classes of equivalent valid phrases based upon the determined degree of similarity between valid phrases.