1. A method of estimating a relevance of a document with respect to a concept comprising: calculating a relevance function of the concept with respect to said document based upon a known predetermined set of concepts in a knowledge base that are related to that concept by different semantic links, said set of concepts being called a semantic neighborhood of the concept, determining whether said document is relevant with respect to the concept and when so determined, calculating an ambiguity function of said concept in said document, which ambiguity function is different from the relevance function, estimation related to by estimating the presence of different meanings of the concept in the document, and marking the document considered to be relevant with a result of the calculation of the ambiguity function.